I'm actually going to say Heimerdinger. He has smarts to help make up for his personal lack of power, though.

Kayle is probably the strongest. The League puts the usual battlefield limiters on her.

orriana

Alistar. He's all upper body, doesn't lift properly.

karma

Rumble

It's mainly because he relies on a suit that barely holds itself together and without it, he's powerless. You'd probably laugh or be incredibly confused if he announced that he was going to fight against trained soldiers, mages armed with forbidden power, and unknown horrors with that thing. If that was to represent what yordle technology can really do, then you'd probably run away really fast from anything made by a yordle in fear of it blowing up in your face.

The only reason he was even allowed in actually was because he hated Heimer so much that when his suit got one-shotted, he was willing to take on Heimerdinger piloting a mecha ten times Tristy's size with his bare hands.

I would say either Teemo or Rumble, without that suit...Rumble can't do much.

Kayle, Jax, and Karthus are top tier.

Singed (more mad scientist then actual fighter) is possibly the weakest.

Edit: Forgot Viegar, old lore plays him up as extremely ineffective and kinda stupid.