Mumble.
I rent a Murmur server from Multiplay.
The best VOIP quality by miles and none to minimal voice lag compared to anything else I have used. If you ever get bad quality on Mumble, it is likely owing to a badly configured server or just high pings.
Saying that, any of them on a well configured/managed server and I would suspect they would be similiar.
We have a ping of 14 on our server and overall quality is EXCELLENT.
As allready mentioned, no need to push to talk either, very good setup options.