As a kid growing up in the 80's, it's only natural that TMNT will always be part of my childhood favourites. So when I've recently watched some SDCC 2012 trailers and clips for the upcoming TMNT animated series, I was immediately taken to the design and snappy 2D-esque animation style (although, it could have done without the influence of anime expressions IMHO). I also suspect the story writing might be tamer (in comparison to the 2003 series) to cater to the younger audience, but I'm geeking out anyway.
In whatever reincarnation, Donatello has always been my favourite. I got really nostalgic when they brought back Rob Paulsen to voice the brainy mutant terrapin (fun fact: he voiced Raphael in the 1987 TMNT series). I thought he suits Don quite well! So have some younger-looking Donny and an insert of Leo:
When it comes to range and power, I'll have you know he's my favourite selection in whichever TMNT games too.
A younger Karai, perhaps? I was taken to her role in the 2003 series, and I admit the 2007 movie design is hot.