I have used Teamspeak http://www.teamspeak.com/
and Mumble http://mumble.sourceforge.net/
They'r all good and have grate voice quality. For me ventrillo was the most complicated, its not really that complicated, but compared to the other 2 it was more complicated than they were. Used teamspeak for a while it was really easy to setup and use, has lots of options and was great for use with my friends.
Now i have switched to mumble since i heard so much about it. The good thing about it is that it has a built in, ingame overlay, so you can see who talks, it also has possitional audio, its not supported for all games, but for the most popular ones it is. It makes it that you can hear the person you're talking to from the side he is in the game, also hear him more quietly or loudly depending on his distance.
I suggest you try out these 3 and choose which one fits you the best.