You're going to get a lot of differing opinions. But for what it's worth, while I have the three major programs installed (TS, Vent, and Mumble) to accommodate any group... I prefer Mumble. But I'll fire up any of the three when needed.

Teamspeak and Ventrilo haven't seen much in the way of development for quite some time, except for plugins which are often hit-or-miss in functionality. However, they are still very capable solutions.

Mumble has the most features, IMHO the best sound quality and supports more sound codecs, and does the best job at auto adjusting sound levels (for those times a pesky player decides to start screaming into his/her microphone). I think the only reason more people don't use Mumble, is because the UI is not as pretty and so setup/customization isn't quite as intuitive (though it's not difficult by any means).

Raidcall is another good option, but it will occasionally suffer from lag more often than I would care to deal with. But many people swear by it.

TS3 with local server for smaller groups...free and fairly painless with good results.
Also gives you the option to connect to others if they have available.

Vent has been my preference for many years. TS3 would be my next choice. I can't stand Mumble and have never used Raidcall.