Generally agree with the suggestions here. CGT, LFA, 458, E60 M5 for a sedan (I have an E39, does sound good but needs some exhaust work to really sound amazing esp. inside the car). Pretty much any Lambo or Ferrari sounds amazing stock.
This one just entered my top 10, Ferrari F12:
http://www.youtube.com/watch?v=VzOaJhdmMB0
Stock only.....
4-cyl: Integra Type R (DC2)
5-cyl: Audi TT-RS
6-cyl: NSX-R, 997 GT3, E46 M3 CSL
8-cyl: 458, F430, F355, know I'm forgetting some. C6 Z06 and Maserati Gran Sport sound great stock. AMG V8s are nice, but IMO not the very best, more just "badass."
10-cyl: E60 M5, CGT, Gallardo/R8... basically everything except the Viper lol
12-cyl: Ferrari F12, F50, just about any Lambo