I'm biased with the S2k, but here is my very non-professional opinion on these cars:

350Z - Great power, very fun to drive, great torque, great sound, nice modificationability (I know, goin' George Bush on you ), good price for the power

G35C - I haven't driven one, but this is what others have said: very very smooth, classy, a little more refined than the 350, not quite as quick, sounds great still, still easy to modify. Mostly for looks, but will go when you want it to

S2K - if you get one, get an AP1 for higher revving, or AP2 for newer and better styling (IMO). Not nearly as much power as the 350 or G, not nearly as much torque, looks sexy, girls love convertibles, easy to modify with a lot of JDM parts for it, best handling of the 4, good price for the highest power/liter engine on the market

RSX - looks nice, more laidback I guess? still have good power, but FWD ftl! My friend has an '05 type-s, responds well to basic modifications, has plenty of USDM and JDM aftermarket support, the most room of the group mentioned.

If you are looking for classy cars around $20-30k range, also consider the Volvo S60R slightly used, or the Mazdaspeed 6 slightly used if you want a little more room. Those are INSANE sleepers with over 300hp, 6MT sedans.

If you like the S2K, buy mine ~3,500 miles, $28k