The best and the worst is a Harley Davidson depending on how much baffleage there is between the cylinder heads and the exhaust opening. Straight pipes and compulsive revvers make me want to crush, kill and destroy!
An air cooled desmo with Remus cans sounds awesome.
I once had the "pleasure" of walking behind a straight piped Boss Hoss as the fecal-brained owner coaxed it to life. I nearly ducked for cover, the thing was louder than shotgun. The jackhole had this stupid grin on his face, knowing he just pissed off a couple hundred people. My ears were ringing for a while and all afternoon I got looks of disdain because I happened to be carrying my helmet.
Certainly not all, but alot of the boxer engined beemers sound like a sewing machine to me. I'm not a huge fan of stunt canned inline 4s either.
The best? Either an air cooled Duc with Termis or Remus cans (or virtually any of the 90 degree twins), any of the three cylinder beasts at full wail with a race can, or a narrow angle V with just enough baffling to keep it from being obnoxiously loud.