A writer over at Slate has done an exhaustive series of testing. Why am I not surprised that the best of the bunch was what I use primarily as my VoIP provider, CallVantage.
While I agree with assessment of Packet8 and Vonage, I have had an entirely different experience with BroadVoice, which has been nothing but positive, including using a WiFi phone. This brings the attention to the fact that one person's experience in one location, over one broadband provider will not mean that the next person will have a similar experience. Network topology clearly impacts the experience, which is why the auto-grooming QoS feature of CallVantage sets them apart.
I'm also surprised AOL wasn't in the trial, nor SunRocket, but maybe that will provide for another report by the writer later in the year.