I would say the best, by far, is Dolby Axon, although it is very underused.
(https://axon.dolby.com/)
It just handles multiple people talking at the same time much better than any other program, I would highly recommend trying it.
After that, I would take Ventrilo, Mumble, Teamspeak, and Skype, in that order.
Edited by Kell, 02 July 2012 - 12:19 PM.