Champs you thought were op when you started
And still think are op.

And still think are op.

I've been here since S1 but the first OP champ I discovered was
then I quickly realized
was far better and would get a penta kill with him every game I played him in just about... then they nerfed him... really hard... and dominion came out! and I use to play the old Eve on Dominion! and omg
was made for that map... Then rammus was OP for a while if you had spell vamp runes... but they screwed up the jungle item that made him awesome... amumu was super OP for a while after that... then Tryn... now I play shaco =\
But the most OP champ forever will be 
AD teemo.
I built 5 BotRK and just ambushed people.
Ambrushed?
that is not okay. you sir are a monster. anyone who would stoop so low as to build anything but ludens on teemo more than 4 times is evil.
Graves (season 3). He was the first 6300ip champ I bought because I saw someone wreck with him on Twisted Treeline.

When noob
Years later 
None really. Most the champions I thought were overpowered I since learned to deal with. Evelynn? Learned to vision and safety. Darius? Learned to safety and duel positioning. Yi? Learned what exhaust, Warden's Mail, Grevious Wounds and Kiting do.
I started in S2 preseason. There isn't a champion yet that I think is OP I did back then. Some are OP, due to number issues or overloaded kits, but usually Riot fixes those or dumpsters them until a rework.
But if I absolutely had to pick something. Nidalee, simply because regardless of meta or design she has always been a good pick, because her bloated kit is simply... too hard to balance.

I will never enjoy playing against her when it's anyone that's remotely competent.

I thought killing bots using DoTs were the most OP things ever.