Depends how competitive you want to be. If you're not determined to go national: Miata, hands down. I'd look for a 94-95, but any year would be a good car. They're relatively inexpensive, very reliable, always competitive in their class, and a lot of fun. Also: tons of aftermaket parts, easy to work on, and a lot of great resources for info.
Other good considerations:
Integra: Type R is hard to find, but great FWD car. Kinda ricey.
MR2: again, more rare than the Miata, but mid-engined nimbleness and unique.
'89 Civic Si: actually, a few different old Hondas are good. They'll never feel as balanced as the other cars, but can be competitive in STS for cheap.
Contour: not competitive, expensive to mod, hard to work on, but you get a lot of looks when you make it go fast.

I'm sure I'll post more when I think about it. Check out the SCCA forums for TONS of ideas.