I would suggest "The Drumhead" as perhaps one of the best bottle shows across all the series. And although I don't think it fits in the same category as that, I'd cite "Where Silence Has Lease" as one that's always been a favorite of mine. And, man, that's a true bottle show. Only one guest star, and the only scenes off the Enterprise on an identical sister ship!
And I agree with categorizing "Shades of Grey" as a bottle show, and with its label as the worst.