My most to least favourites are:
Raph
Mikey/Donnie (tied)
Splinter
April
Leonardo
I don't think Leos design is bad by any means, but I agree that he is a bit too lanky/angular looking and the combination of his red marks and creepy expression doesn't help things.
Rise Raph I absolutely love. His design and voice keep on reminding me of Maui from Moana for some strange reason