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.
Vent is my personal choice. However, I have and consistently run both Vent & TS at the same time. I have vent set up as a private server for me and a select few friends and Team Speak (owned by the guild) set up for the 500 person guild I am in. This way I can carry on conversation with personal friends without sharing with random people.