I would say most turbo cars are ruled out. In exception for STi's, clarks supra, hunters SS, Nick and Phil's Evo's. The whoosing and whiring of your typical 240 or stock Evo just doesn't sound like anything fantastic to me..

I.M.O if you've heard a new M3 in person then you'd know it sounds more refined and pleasing to the ears than almost anything on the board. The means if it's not a smooth high revving N/A car or one of a few insanely fast turbo cars then it doesn't even qualify for the best sounding, based on what i think..
so candidates i.m.o...
hydroshutters vette
Ryan's E92 M3 DCT (doesn't post much)
Bokix's M3 (although it's very raw)
clarks supra
The Youngn's Cayman
on 3's M6

Also dirtyMechanic's G35 sounds nice...

It's hard to say because the board is filled with Honda's and 240's and all similar domestics and all the different individual cars sound the same as each other. Some civics are loud, some arn't as loud...they all sound similar. There isn't that much variety or any exotics....
It would be a given if someone had a V10 M5 with no cats or even a Carrera S or E39 M5 with full exhaust. One of these would sound the best by far, but nobody really has a different/expensive car like that, that posts.