Old Vekoma coasters have a great "whine". It's not just limited to their corkscrew coasters, both boomerangs and SLCs have the same noise, just a little quieter.
[youtube]http://www.youtube.com/watch?v=p9tl-gUq51c[/youtube]
The anti rollbacks on Arrow Coasters is a sound that I adore. Just sounds so loud, like metal randomly slamming against more metal at irregular intervals.
[youtube]http://www.youtube.com/watch?v=u7YO5zN_rPY[/youtube]
BTM at DLP is definitely the loudest that I've heard, for sure. The don't seem to be traditional anti-rollbacks with "teeth", more like rollers I guess. The sound really adds to the atmosphere in my opinion, except for the ones that they've added around the course of the coaster in recent years. They just sound horrid when the train teavels over them at speed.
[youtube]http://www.youtube.com/watch?v=mLZRIN0PUFE[/youtube]
Colossus and Nemesis also spring to mind when cresting the lift.