This is a fairly subjective question - I can only provide subjective responses :-) but I've used all three (Skype, GTalk, WLM) a fair amount.
Skype's SILK codec is vastly improved over previous iterations but for me - even with over 2 Mbps of upstream bandwidth - it still takes a while to scale up to use all the available bandwidth, then it seems to want to lower in quality at the drop of a hat (or lost packet).
GTalk's quality could in theory be excellent; their plugin supports:
but in real life I've only ever experienced the pleasure of a 16 Kbps Speex call. :-( Windows Live Messenger is pretty much the same quality wise - Skype edges it. However, Google Plus' Hangout (which uses the latest Vidyo codecs) is (at the moment at least) really good quality and pretty much on a par with Skype for pure audio quality. Looking forward to testing more as times goes on.
Skype is also quite tolerant (so it seems, from a weekend of testing in a hotel with overpriced wifi) of 3G connections, at least with a VPN, HSDPA and an acceptable level of contention on the cell tower. to get a call for more than 3 seconds without almost 100% dropped packets I had to VPN through the connection (as expected, as I'm on the middle tier for my mobile broadband package it deprioritises Skype traffic). I had very acceptable latency of about 200-250ms; much better than I thought. Throughput was fairly steady too.
So at the moment, Skype pips it for call quality - but I very much expect Google/Vidyo to surpass that quality, particularly given the SQ and PQ of calls I've seen over standard broadband using the standard Vidyo-branded service.