The others all have cool personality quirks or tech skills, but let's be real: if the entire planet is in danger, and I need the BEST turtle to have a shot at saving the day, I'm going with Leo. He's the most even of them all. He's got the kindness of Mike, the ferocity of Raph, and the general intelligence of Don. All the others can do one thing very, very well, while Leo does pretty much everything well.
Like, depending on the situation, if I need a bruiser, I'll go with Raph. If I need some science to save the world, I go with Don. If I need to win a sick-ass break dancing contest, I go with Mike. But if all you tell me is some shit is going down in general and I need help, you bet your ass I'm going with the most serious turtle. The one the others identify as the leader.