I don't know about definitive production, but Tracy Letts was the definitive George for me in VIRGINIA WOOLF
I could not agree more. I think the production was probably the one that made Martha least monotonous. Uta Hagen was good, and Elizabeth Taylor certainly got the screaming down, but Amy Morton gave her some colors. But Letts was astounding. I would even go so far as to call that production definitive.
For me, it's the original GREY GARDENS, which was a perfect evening of musical theatre, exquisitely directed and designed (By far, Michael Grief's finest work.) and with unsurpassable leading actresses (Wilson, and especially Ebersole). It's a wonderful piece of writing, and I'm glad it's being done a lot and that new performers are getting a chance to explore the parts, but I don't think I'll ever be able to shake my memories of that original production.