I've actually driven most of them. My impressions:
MR2 - Loved it, but there's NO trunk room
Mini Cooper - Also liked it a lot, probably the one I'd choose for the price
S2000 - Great car, but impossible to hold a conversation with the person next to you, unless you're both shouting. Plus it's a Honda
Miata - Old standby; awesome handling, but crappy vision. I fell like I'm gonna die
Z4 - Do you even have to ask? Best of the best. And not obnoxiously expensive, either
Crossfire - Cool, but overpriced. Same for the SLK (except for the new SLK 350)
350Z - Buy this car. Period.