Mumble is easily a recommended client, but I've grown tired of having 4 different clients installed for every voice chat I'm in. I've recently opted to make a
RaidCall group. The client is free, the server is free, it has Youtube embedding in the chat, easy sound setting management with the ability to use foobar2000 for music playback. Channel management is a tad weak with no sub-subchannels (subchannel within a subchannel in a channel) but administration and moderation are effective, and the UI is comfortable enough.
You have to register an account, but this means less mucking with IP addresses, and wasting internal computer resources to manage a server.
Though, it comes to preference. Some want Mumble, some Ventrilo, some TS3. It's hard to please everyone.
EDIT: I got more pictures, including stuff like the queue, and announcements.