I don't have any experience with anything except Autotune, which I found was fairly miserable in manual mode. Worked fine automatically, but the manual mode sucked.
Lately I've been using GVST's FREE "GSnap" plug-in. You can use MIDI to zero in on bum notes, or use it in automatic mode. I can't lie and say it's the 'highest quality available', because I honestly don't know. I'm using it because I have access to it, which is something I can't say for Melodyne or whatnot.