I like OB-Tune, as well, but it is limited as a 32-bit DXi plugin. Unfortunately, it is (and hasn't been for a long time) available any more.

I've tried GSnap, but like Warren, I was not satisfied with the results.

Melodyne does a great job, but it also generates some weird artifacts at times. I have not had great success using it live on a track; I usually offload the WAV file and use the standalone, set the parameters to bring everything more or less into tune, but not 100%, and then edit note by note as needed. That makes the end result less robotic.
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John

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