I'd have to think on all of them, but a couple stand out.
Patrick Stewart - "Sarek". While it's easy to say "The Inner Light" or "Chain Of Command, Part II" or any number of others (and he had a LOT of great ones), I might actually say his best was after the mind meld when was alone with Beverly. He went through a wide range of emotions in such a short time... and convincingly, too. He was robbed of getting at least an Emmy nomination.
Jonathan Frakes - "Frame of Mind". He was an actor who just 'went for it' with everything... from being engulfed in metamucil and printer ink to being experimented on by aliens. And while his scene with Worf in "Ethics" almost wins for me, his performance in the entire episode here was just excellent. He really sold us on what he was going through. One of the best episodes of Brannon Braga, too. And James Conway is always a great directing choice. One of my favorites of TNG, period.