Seriously. One that feels good, that you're willing to get on and be seen in; one that FITS - so go to ye olde bike shoppe.
Though -- I'd look for less urban spots to get out and cruise. It's worth driving a little bit to find a place where you can ride wtihout fighting traffic and what have you.
I ride a hybrid because I'm not happy bent over (there are back muscles I just don't own), or a venerable Schwinn coaster brake special which I'm sure has half the town convinced I'm homeless.