Solo? Get a A class (expensive) or a F16 (more affordable)

Duo? Depends on your weight with crew. F16, F18 or an N20 if you want to go far.

Strapped for cash, just want to have fun? Get a frankenboat, any old Nacra can be made exciting with a spi, a H16 is always a sure bet and take a spi (but not too much weight).

Franckly this is quite an open question, everyone is going to jump in with recommendations. Without information about how you are going to use your new toy it's hard to recommend anything. Find the nearest sailing club, go see what they sail, buy one there used from one of the guy, go sailing.