I voted for mumble, as from the brief amount of use I've got of it, it seems to maintain the lower latencies I've seen from teamspeak, whilst not being that system heavy, having a nice room ingame overlay, and also having decent voice quality, think it also has built in normalisation on voices which is nice.
Haven't tried TS3 to see how its improved from TS2, but that'd be my second choice...TS2 sounds worse than vent, but its lighter on your system and lower latency in most cases, which is more important than sound quality as long as you're understandable.