Leonardo has the best color (blue), the best weapon (sword), and he is the leader.

Donatello is smarter than Leo but Leo could cut his weapon in two pretty easily.

I'm not really sure what Raph and Mike add to the group other than comedic relief.

Case in point: in the original NES game, Leo was the best turtle by far. His weapon had the second best reach and second best strength, only beaten by Don's staff (and Leo's sword was much quicker). But, the huge advantage Leo had was that his sword swipe could reach smaller enemies and flying enemies without ducking/jumping, which Donatello could not do. Raph and Mike could also do that but the reach on their weapons sucked.