S1 - Nunu had best buff/debuff combo + insane base damage on abilities as a support

Alistar was a mid laner who was tanky, had sick sustain from heal and one shot people with combo + lich bane auto

Rumble had too high damage numbers upon release despite people not having him mastered mechanically yet

S2 - Ez was op, because W attack speed buff/debuff wasn't removed from it yet

Jayce one shot people, because he had too much damage with melee form (E was maxed by then for some insane % dmg numbers)

Shen was always strong for the same reason, same goes for back then. He didn't have as good of a kit, but there were far less threatening top laners

S3 - Zed was maxing E back then and he basically put down his W and E'd you twice before the shadow disappeared for free lane win against every match up in the game

Shen - Same as S2

Corki - Because Ez got nerf on his W and Corki got buffed with trinity for worlds, again

S4 - Ali, Zil and Mao were straight up OP, while players got to high enough of an average skill level to handle Lee properly

S5 - Juggernaut + GP reworks and Elise which should've received a major nerf for a long time, yet still didn't

S6 - Nida was OP ap jungler, because of the sick itemization. Syndra was pretty much as strong as she is right now, but with way less opposition and Olaf fit both the tournament META and got enabled by the items in similar fashion to Nida