subaru's do sound great, but it gets a little played. imo they only sound "great" when its a rally car, the exhaust, mixed with the turbo whine, backfiring, and the blow off sound all mixed together is awesome.
imo the best sounding 4 cyl car is an e30 m3, especially the race cars.
otherwise, ferrari v8's that zonda, and the carrera gt and especially the morgan aero 8 sound the best for street cars.
and you have to admit that american cars do sometimes sound great, i guess its the only thing they can do well.
for any car though, id have to say the turbo v8's in champ car's. the blow off sounds like a gunshot or explosion every time, that mixed with the slightly deeper formula one sounding sound and the backfiring is so awesome.