I jus bought Katarina after playing her the last time she was free (and it was super fun!) but now, holy god. Katarina's last name must be Sucky (initials would be KS), because killstealing is the only god forsaken thing she can do, and she isn't very good at it either! I can jump in 3 levels ahead of people and use all my skills, to watch them (annie, veigar, nunu, shaco, TF) use one skill to drop me to 1/3 hp.
/If/ TF isnt op, next would be annie imo. I have yet to see an annie not get a 8/1 k/d spread. 'Stack MR!', but that cuts out your own dmg and its expensive usually, while annie can buff her attack AND get pen, in addition to her low cooldown and naturally high dmg. But TF wins, just because his global teleport and more dependable stun.