After hearing the Merc in Leno's http://www.xoutpost.com/off-topic/lo...st-famous.html video thought it would be cool for people to list theirs along with a clip if available -

Since this is an X5 board and one of my favorites I'll say X5 4.8is with resonator delete/X-pipe (although some of the other exhausts sound great too, here is a great link
http://www.xoutpost.com/articles/x5/...deo-clips.html

Ultimate car sounds -

Part 1
YouTube - Ultimate Car Sounds, Part I

Part 2
YouTube - Ultimate Car Sounds, Part I

Part 3
YouTube - Ultimate Car Sounds, Part II

Part 4
YouTube - Ultimate Car Sounds, Part IV

I think I'm partial to the Aston's, Ferrari's, Ford GT's, ZR1's and Lambo's.