See poll.

I will have to go with Mumble. You will need a server of your own I think, but other than that, it has lots of features. Definitely better than Skype.

It has a pretty well customizable overlay interface plugin thing that works with certain games using Direct X 9 (or something), you can really customize all sorts of options for button-to-hold-for-talking or automatic talk like threshold and whether they key or keycombo is reserved exclusively for Mumble while it is running, and options for organizing people into rooms like Vent, among other things such as bandwidth usage, sample rates, compression, and so on (good for if you have shitty Internet like me).

In short, amongst Skype, Mumble, and what I remember of Vent, Mumble is the best. For me at least. The customizability might be scary for computer noobs I guess. With that said, Skype and Vent are how-i-type-fast and p-decent tier.

I've been using vent for years. Nearly every clan I've been in used vent.

I have Teamspeak, Mumble, Ventrilo, and Skype, but of those I've personally found Vent to be the best. I like the idea of Mumble's overlay in games sometimes, so that's a plus as well, but I found it to be distracting when I used Mumble and ended up disabling it.

I like Ventrilo best overall cause it's very easy to use. Definetly the best choice for a smaller group (never tried it with more than 10-15 people).
Teamspeak is a bit more advanced and seem to be more used by larger groups. Pretty much every alliance I've been in on EVE have been using this.
Skype doesn't really work if you're more than five people. Also it's lacking a lot of customisation and you have to make an account to use it.

We use mumble at work sometimes, but I couldn't really see it in a gaming application. Definitely wouldn't want to be gaming with skype. And last time I tried to download teamspeak (albeit this was probably six years ago) it was a colossal pain in the ass. Vent is easy and small and almost every guild I've been a part of uses it.

I prefer whatever is the most lightweight and efficient. Which is vent, in my experience.

Nude, what "other" program do you use?

I like Skype the best. I use to chat/talk to people, and I'm never really playing with more than like 5 people, so it works well for that. I used to use vent, but it seems like the people I used to talk to hardly get on anymore. D;

telephone

Skype because the people I would voip with are already there and it does what I it to do, which isn't much.

Teamspeak 3, because of its power. Amazing permissions system, great mods and plugins, user friendly for computer literates, and is the most bandwidth conscious program Ive seen in years.