Well, in context of the movie and cartoon, I'll use both
1) Strength: I'll be a biased fan and say Raphael, he had some temper issues, but he seemed to have some good power in his muscles.
2) Athleticism: Mike seems to be the best candidate for this, always sewer surfing or skate boarding.
3) Intelligence: Donatello most definitely, the brains of the four. Leonardo
also comes off as very bright but he leaves the thinking to Donny. I think Raphael is quite intelligent too, but he chooses to be sarcastic rather than logical.
4) Level Headed: Leonardo most of the time...Splinter's ninja teachings seem to have taken heart best in him when it comes to concentrating.
5) Fighting skill: All four Turtles have almost equal amounts of skill per in my opinion.
6) Best overall fighter: Raphael
7) Best overall turtle: Raphael