For me now a days it's any 6 cylinder car with a good exhaust, and Subaru's. One other I can't lie about is an un-corked Rotary, damn they sound fantastic.
Triumph TR6, 6 to 1 and a super trapp
Porsche AIRCOOLED flat six with megaphones, or any sort of cat delete. I prefer the N/A sound, but the turbo's are fantastic as well.
Supra's
Toyota 3.5 V6 with custom manifolds and mufflers
NSX with tubi's
VR6
Plus add ITB's to some of them and they get even better.
RWB Porsche
Zork Tube 935 - which is basically a short exhaust pipe straight off the turbo.
MOAR RWB Porsche, it's this little shop in Japan that builds these fantastic Porsches out of the same warehouse they started out in years ago.
http://www.youtube.com/watch?v=KW3led-DtY8&feature=related