Currently the only slightly overpowered champ would probably be graves. The AS increase on his dash is pretty ridiculous mid-late game (80% herp), and his base stats are slightly too high. other than that he's pretty okay.
Tryndamere isn't remotely powerful, especially after his remake. He's just a pubstomper, thats it.
Akali is pretty strong in this current mobile-assassin meta, but with the gunblade nerfs I think she's leveled out enough (even though she's 1st pick/ban quite a bit :v)