I've used several of them, but (for me) Ventrilo is the best. A *lot* of people seem to use Teamspeak, so I guess its the default 'top dog'. But (after much fiddling with the controls) ventrilo tends to give me unbelievably clear audio and it doesn't seem to affect my ping/framerates while I'm in game much at all.
I've actually put up a ventrilo server on the webhosting account I use to run my gaming group's website. It was pretty easy to set up.