Competitively, I think it'll probably be Donatello, because he has a (seemingly) better character power that can mess with different opponents and I see that bo staff anti-air being VERY popular in the Tourney scene.
I could be 100% wrong on that, though, but just my hypothesis.
For this pleb, who loves playing as his favorite Turtle and is nowhere near Tourney level, Mikey and probably Don in close 2nd, because I'll probably be undecided for a while after yesterday's stream.