There are too many to choose.
The Mclaren F1's BMW V12 makes a beautiful sound, especially racing versions with straight through exhausts.
TVR Griffith De-cat
Mercedes Mclaren SLR, with the deep, gruff blat of the V8, combined with the high pitched supercharger whine. (does anyone have any MP3s?)
Flat plane crank Ferrari V8's, the 355 De catted sounds more like an F1 car than anything else on the roads.
BRM V16
Rolls Royce Merlin (you didn't specify car engines)