Here are my criteria to determine which champions were the most impactful:
1) Longevity - Surviving for multiple metas is the most important attribute for this list. If a champion can survive for multiple metagames, then clearly they must have had a large impact.
2) Peak dominance - If a champion is pick/ban for a metagame, this will make them rise on this list. This is where the interchange of peak dominance and longevity is mixed up. In my opinion, it's better to be a solid pick for many seasons than it is for a champion to be pick/ban for a short while before falling out of the meta due to counterpicks or nerfed. A different valuation would lead to a different list.
3) "Innovation" - Obviously it's not possible for a champion to be innovative all by themselves. What this category means is a champion will be moved up in the list if they changed the way people think about the game. One of the most prominent assassin mid laners was Zed. Zed (along with Kha) helped shatter the mage metagame during the preseason of Season 3 and returned to once again define assassin play during that same season's World Championship.
4) Uniqueness - The final important trait is uniqueness. If a champion could do something that no other champion could do at a high level, that will impact its placement
Honorable Mentions:
Ashe was the first extremely powerful mid laner in Season 1, when players sent AD champions into the mid lane. Ashe's status as a utility AD Carry showed a hint of what the mid lane role would soon become. Brought to life by the skilled hands of Fnatic's Shushei, mid lane Gragas was once one of the strongest champions in the game, a deadly hybrid mage/assassin that could burst opponents to the ground while providing respectable utility. Gragas is one of the best examples of player-driven innovation - Riot had intended for Gragas to be a bruiser like he is now after his rework. Gragas's deadly combination of burst damage and utility made him a key pick in Seasons 1, 3, and 4 until his rework. Finally, Lee Sin and Jarvan IV, played near the end of Season 2 and in pre-Season 3 helped break the idea that the only mid lane champions should be mages. Although bruisers never truly took off in mid, these champions helped players to think outside the box and eventually bring lasting innovations such as assassins or the return of AD Carries. In Season 4, Ziggs had some of the highest game impact ever due to his minefields and global ult, meaning he could stall games nearly indefinitely.
One of the more recent additions to League of Legends, Azir has been a powerful mid lane pick ever since his release. In spite of repeated nerfs, Azir utility is just too great for teams to ignore. With strong poke, high tower-taking potential, and a strong disengage ultimate, Azir has one of the most complete kits out of any champion. We'll have to see if the removal of his knock-up will dethrone the new king of the mid lane, but his performance and popularity thus far have been through the roof.
What keeps Azir from rising higher on the list is both his relatively short longevity compared to champions that have been around longer and his lack of historic impact - Azir simply fit into the mid lane metagame without changing how players think. Funnily enough, these are problems faced by newer pro players climbing up for greatness as well. A player like Bjergsen has had a better Season 5 than Alex Ich, but Alex's innovation and longevity have played a significant part in his greatness. Azir is in a similar situation.
During Season 2, LeBlanc was known as a popular pocket pick for aggressive mid laners like TSM's Reginald or SK's ocelote. However, LeBlanc never caught on and was generally overshadowed by champions like Orianna for the majority of Seasons 2 and 3. When LeBlanc finally returned, it was in the hands of the greatest player of all-time: Faker. In his debut game, Faker would lock in the little-used Deceiver and destroy Ambition, then considered the best mid in Korea.* For the rest of his career, LeBlanc would remain his most trusted champion, and he was undefeated on her until he met EDG's infamous "anti-LeBlanc comp." Even against such a dedicated strategy, Faker and LeBlanc fought together nearly perfectly.
However, LeBlanc's greatness extends farther than her status as Faker's queen. After her rework, LeBlanc was one of the most popular mid lane champions throughout Season 4 and retained her high status during Season 5 until a nerf to the speed of her W.
*This was an error, Faker actually played Meijai's LeBlanc against MVP Blue, later Samsung Blue. Against Ambition, Faker played Nidalee. Thanks to Ceui for noticing this mistake.
Ezreal lacks the competitive results to climb higher on this list. Although he was played in mid lane ever since Season 2, the Prodigal Explorer has seen very little competitive success. Although teams will bring him out for regular season games, they have almost never played him in the play-offs. (Mancloud and dade are some of the most prominent Ez supporters, and are two of the few players to use him in the play-offs.) This season, AP Ez mid with Runeglaive looked to be his best shot at seeing competitive play during the play-offs, but the Runeglaive item was nerfed and it looks like Ez will once again fall out of popularity before play-off time.
It's interesting that AP Ez was the build path most likely to see play-off play, because the main reason Ezreal ranks so high is his role opening the door for AD mids. Both Corki and Varus have seen more play-off usage than Ezreal, but pro players may not have considered playing those champions in mid lane if teams hadn't initially broken the mold with Ezreal mid in Season 2.
Nidalee saw a big of play in the mid lane near the beginning of the Season from assassin players like Bjergsen or InnoX, but her mid lane career has mostly been over after her rework in Season 4. When players were focusing on utility, Nidalee was unique in that her primary focus was mostly poke damage. Her utility more or less made Nidalee the perfect poking machine. A powerful AP scaling heal made it virtually impossible to defeat Nidalee in a poke battle. To make matters worse, her traps helped Nidalee keep her flanks covered to prevent enemies from engaging on her team with a surprise maneuver.
It took a while for Nid to catch on, as she was mostly a pocket pick until the Season 3 World Championship, but from then on until her rework she was easily the most threatening mid laner and a must pick or ban champion.
Karthus is one of the oldest mid laners in the game, and his influence on competitive LoL history has matched his time as a playable character. In Season 2, the mid lane was dominated by mages which farmed for late game teamfights. Although players had their signature champions, such as Anivia for Froggen or Lux for RapidStar, every single player simply had to know the Deathsinger. In the Season 2 World Finals, the team that had Karthus won every game until Azubu Frost banned him for the decisive Game 4, deciding to give Toyz his beloved Orianna instead. Since then, Karthus's star has fallen with the rise of assassins, but the undead never truly die. Players have tried to bring back Karthus in every season, and he remains a pocket pick for legendary mid laners like Froggen and Faker, both of who went so far as to call Karthus a counter to LeBlanc.
However, this same dip in popularity is what keeps Karthus from climbing higher on the list. Karthus defined a single World Championship, but the champions in front of him defined multiple.
In Season 2, TF's global ultimate meant that he had unprecendented playmaking potential from the mostly stagnant mid lane mage meta. However, his low cooldown Q and W meant that he could still go toe to toe with the same mages in mid-game teamfights. Even with assassins coming into vogue in Season 3, TF remained insanely popular, especially in the Asian regions. TF had a 100% pick ban rate in OGN Champions from the end of Winter 2012/2013 to a group stage game between IM2 and MVP Blue in Summer 2013. Even though he had an awful lane against assassins, his unique ult was too hard for teams to deal with.
After some nerfs, TF would fall from popularity as a dominant meta pick, but he's never truly left the game. He has always remained a deadly pocket pick for experienced players, and the current mid lane meta, with its relatively low kill threat lanes, is looking like a great chance for TF to shine once again.
Fans always joke that Ahri, Corki, and Jax always come back for the world championships, and this year it looks to be no different. With Worlds just around the corner, teams have once again begun experimenting with Ahri, using her in pick comps alongside supports like Thresh. The defining trait of Ahri's longevity has been her versatility and a seemingly concerted effort by Riot to make sure that she stays a top-tier champion.
When Ahri was first released, her high base damages meant that players could build Abyssal and Rylai's to make her a very powerful tanky damage dealer. Ahri's play saw a brief dip after those items were nerfed, but she returned in full force during Season 3. Once again, her high base damages meant that players could use DFG to burst down opponents even if they missed her charm. To try and curb her dominance, Riot gave her a mini-DFG on her charm but lowered her base damages. Ahri stayed powerful, and a subsequent change to remove the mini-DFG in exchange for movespeed on her Q turned her into a must-ban kiting mage. Ahri has pretty much seen it all from the mid lane, but she still isn't as dominant as the 3 champions ahead of her on the list. She hasn't had as large of a transformational impact, and generally fits into meta trends rather than starting them.
Every time Kassadin comes back into the meta, he almost immediately becomes a must-ban champion. One of Kassadin's earlier moments in competitive play was also his finest - xPeke's infamous backdoor against ocelote's SK Gaming. At Season 3, Alex joked that Kassadin was Europe's secret weapon, and both he and fellow European legend xPeke were able to play that champion to great results. After other teams saw their fearsome Kassadin, he became a must-ban champ for the rest of the tournament...and for nearly the entire first half of Season 4.
Perhaps the only thing holding Kassadin back from climbing higher is his own strength. Because he is banned so often, we rarely see great Kassadin play when he is strong. When we do see Kassadin, it is often after extensive nerfs and the champion has inconsistent performance outside his main meta. Because Kassadin defined pick-ban across several seasons, he deserves the third spot on the list. However, the top two spots go to champions that have not only been must-pick worthy but have stayed extremely relevant when they fall from OP status.
Zed completely revolutionized how players viewed the mid lane. After Season 2, players believed that the mid lane should always be oriented around late game farming champions. Many players, including Season 2 World Champion Toyz went on record claiming that assassins would never beat out mages because they would always be outscaled. Zed brought a package never before seen in assassins. He had a spammable ranged spell to help farm in lane without too much punishment from ranged champions. He had unprecedented safety with his shadows. But most importantly, players finally realized that if an assassin zeroed out a player, even a support, their teammates would be able to immediately force a fight due to the 5v4. Teams went out of their way to snowball their Zeds and Kha'Zixes at the beginning of Season 3, and the end of Season 3 was no different. Throughout the Season 3 World Championship, Zed defined pick ban.
Outside of his dominance at Season 3 Worlds, Zed was also a very important pick during Season 4. Afterwards, he has remained a relevant pick but has seen a dip due to the nerf on his ultimate which doesn't allow him to "shadow back" for a full second after using it. Zed has longevity, a lasting impact, and even two parts in one of LoL's most famous plays - the Faker vs. Ryu outplay. There's no doubt that Zed is the king of the mid lane.
Zed might be her king, but the mid lane has been a matriarchy ever since Orianna's debut in Season 2. The Lady of Clockwork has unparalleled versatility and strength. Orianna's high damage and utility has meant that ever since her introduction, she has never been irrelevant. In the mid lane mage meta of Season 2, Orianna was able to match her opponents in farm and outperform them in teamfights with her ultimate. In Season 3, Orianna players used her shield and auto-attack damage passive to go toe-to-toe with assassins and even kick them out of lane. In Season 4, Orianna's high poke damage and waveclear kept her relevant in a meta filled with rival poke champions like Ziggs or Nidalee. It also didn't hurt her case that Orianna's ultimate helped supplement engage and break sieges. Season 5 has been her least notable season so far, but she has returned to the meta once again in a mid lane which surprisingly resembles her debut - mids such as Azir or Viktor focus on wave-clear and farming up for the late game.
Never irrelevant and always deadly in the hands of experts, Orianna has rarely been banned but almost never goes through a BoX unpicked. With her recent return to form, it doesn't look like this will change any time soon.
Reference : https://gamurs.com/articles/the-top-10-mid-lane-champions-in-competitive-history
