My personal favorite on the list is the Fiero (laugh here). They're cheap and most parts are easy to find. The harder to find parts are mostly on the 88's since this was the first and only year they had a new suspension design. But there are plenty of aftermarket parts for these cars and with a little work an early fiero (with the chevette suspension) can be made to handle better than an 88.
The best recorded number ive seen so far is 1.14g's on the skidpad with a mildly modded 88 GT. This car also has a Lingenfelter corvette V8 in it. (A few pics can be seen in Hot Rod Mag. from a few months ago)
Also, just about any GM motor can be put in them. I've seen them with Quad 4's, 3.4's, 3.4DOHC, 3.5DOHC, 3.8, 3.8 S/C, 350's, 405's, 455's, and Northstars. I've also seen pics of a GN motor in one, and a 1st gen DSM motor in one.
If you can't tell, I know a lot about these cars and hope to have mine by this summer if all goes right.