I mainly pick characters based on their design, if they have a great movelist that is a plus, otherwise i try to put the extra effort and time in to learn them. Looking at their fighting style comes next, if they have interesting combo's and such. In SSFIV for instance i can't stand alot of the characters mainly because they look goofy and are very unintersting, examples:Guile, Zangief, Blanka, El fuerte, Rufus, Sagat. The way i pick characters is probably not the most efficient but it's what i like.
DOA4: Elliot, Hayate, Ryu Hayabusa
SCIV: Setsuka
SSFIV: Ibuki, Cody,
Tekken 6: Miguel, Leo
MvC3:X-23, Dante, Dr.Doom
MK9: Noob, Ermac, Sub-Zero(looking to main Skarlett also)