My favorite Sonic is Mari- wait crap someone already made this joke!
My actual favorite Sonic generally is the way the Japanese portrayed him in the Classic era. Can't go wrong with sticking to the original design that the creator made.
Although if I had to chose between the designs in the 3d games, it'd be dreamcast Sonic (specifically the original SA1 model.) He just sorta seems like he could realistically roll into a ball, sorta keeps the pudgy-ness of Classic Sonic, and is built to look good as a 3d model, unlike the classic design.
Also, this thread really accentuates just how fractured the fanbase is. I sorta wish Sonic wasn't as fundamentally aesthetically pleasing as he is, so that we could be like the mario fanbase, where most other designs besides the main one look terrible and aren't liked by the fandom. Ninty-percent of Sonic designs are at the very least meh to look at, which, since there are a lot of them, means that most are pretty good. Seriously, think of three Sonic designs that you don't like. If I were to say that two out of the three of those you simply don't prefer, but would still say are good, I would 90% of the time be correct in that assumption.