uTorrent is a good program, small, low impact and with plenty of features. Its the one I use.
Azureus give the best speeds IMO but only slightly better then the others and it is a pig! Its a Java app. and devours memory, CPU time and even some of your bandwidth. Use it for a long time but switched cause of its flaws. Not for a low end system.
Bitcomet is passable but is the weakest of the 3 mentioned. Its not *bad* mind you; good speeds, nice interface, average system usage, etc etc. Its just not a winner in any of these catagories. Its one of the most commonly used torrent programs out there.