Really depends on what you're trying to get out of the machine. A pleasant Sunday cruise in a comfortable car or something more spirited and sporty?
Honda S2000. Excellent handling and balance, cool dash, a bit rough riding (as expected of such a car) but a high strung car that you have to rev the crap of to get real performance. Other than that, it feels like driving a well built, reliable Civic.
Any of the British classics (gorgeous machines mind you) and you spend more time working on the thing than actually driving it.
Mazda MX-5 and VW Beetle Cabrio. Good reliable, well built, fun cars (especially the Miata), but totally a chick car.
BMW Z4, Audi TT and M-B SLK. Higher end, more powerful and faster interpretations of the above. Still totally chick cars - sure the "men" who drive them will argue this point, but deep down they know.....
Ford Mustang. Big, heavy, sluggish (V6 anyway), handling is years behind the Euros. Low quality interior materials, but is reliable and cheap to own. Get a white one and it becomes a chick car though.
Saab 9-3, Volvo C70, BMW 1/3-Series, M-B CLK, Audi A-4 Cabrio. All good, well built, fast, comfortable weekend tourers but lacks the harder edge of a true sports car.
My pick. Used Porsche Boxster (even better the "S"). Everthing a weekend sports car should be. One of the best balanced cars around, more than enough power, great driving dynamics. Only downside is probably insurance and maintenance costs.