350z - nice rwd vehicle with alot of potential imo
sti - all around niceness, awd, turbo, 4 dr
s2000 - one of the best roadsters i've seen in while. its a 2 seater tho so if you're a space lover then don't bother
imo it all depends on what you want. if you are more partial to the 4 dr then go with the sti, but if you want a coupe then i would go towards the 350z honestly.
all of these are fairly pricey, if you would like a quick ass car for around a 20g price range i'd go for a 2002 + BMW m3 maybe.