From an admin perspective. I'd have to go with TS3+ the NPL license allows you up to 10 virtual servers, with total slots of 512, this assuming you're running on your own server. I've had mine loaded up to 200 users, and it's been 100% stable. I think most TS3 servers problems are actually configuration issues on the server side(i've only dealt with my setup on centos5). This combined with the user/client/channel customizations and file transfers makes it quite versatile(especially when considering PUG Admins, testers, etc, etc). It's still not without its bugs. I think users who aren't as savvy, or haven't used voice clients before may find the amount of settings/customization a bit overwhelming.
Mumble is nice, simple, low latency, and decent quality. I haven't had the chance to use it other than as a client(for a few hours), so i don't know the benefits of the administration portion.
Mohawk Voice is another VOIP app with simplicity, quality codec, and low resources in mind. I've only had a few hours as a client on server, so i can't comment much.
Ventrilo is the standard, so there's not much to say, it's what people are familiar with, stable(for the most part). I see issues with ventrilo when running high count of clients. It's quite easy to take down a Vent server and/or make it unusable. Dev has been slow on ventrilo for quite sometime.
All that being said, my vote is for TS3