The mechanical banshee howl of a classic Triumph triple coming up on the cam is a sound like no other. The hair on the back of my neck stands up just thinking about it (which it is doing right now as I type this)
Another nice sound is a pair of DG pipes on an RD400 at idle, sounds almost identical to a top fuel dragster idling up to the staging lights...
(for what it's worth, I own 2 Tridents, and have owned 5 RD400s. I still have both Tridents in my garage, although "technically", I gave one each to my older boys. I gave my last RD400, the one with the DG pipes, to my oldest son who still has it)
Other nice-sounding bikes I own or have owned include a Honda CB550 with full Pops Yoshi gear, and no muffler on a 4-into-1 collector; pretty wild scream. My modern Triumph Legend triple with TORs sounds pretty cool (although not quite as cool as a classic Trident). The D&D pipes on my '83 Honda VF750 sound nice, too...