Just off the top of my head/gut reaction, so I reserve the right to change my answers.
Patrick Stewart -
Generations. For two reasons. The shell-shock he carries in the early scenes, and the reveal scene with Marina. Patrick can sometimes bring slightly too much to an emotional scene, where it almost borders on ham (but without crossing over) and for me, his emotional delivery during that scene is on the money.
Jonathan Frakes -
Second Chances. Nothing too showy, but I appreciate the subtle differences he tries to bring to Will and Tom.
LeVar Burton -
Identity Crisis. Again, maybe not the episode that displays his range at its best, but I think he does solid work and some good no-lines stuff after fully transforming. He sells the hell out of that hug at the end without speaking or really even being easily seen.
Denise Crosby -
Yesterday's Enterprise, thankfully. (Although it's not like she's outstanding in it.) Otherwise I'd have to pass.
Michael Dorn - It's hard to pick a TNG, my thoughts are overshadowed by DS9 choices. Maybe
Sins of the Father?
Gates McFadden - Gates doesn't get a lot of stellar eps (or great moments in other eps for that matter) so I'll go with
Remember Me, simply because she obviously carries that episode, and well.
Marina Sirtis - Marina also gets the short end of the stick, but
Face of the Enemy let her do her thing. Honorary mention for an odd choice, Gambit part I. Her scene with Riker where she's gentle with him at first but then ends up having to tear him a new one for being so selfish.
Brent Spiner -
Brothers lets Spiner do his thing but by virtue of being Lore and Soong, his tendency to go over the top when not playing Data doesn't impact the episode.
Wil Wheaton - Hrm. It's hard. I don't want to dunk on Wheaton, but I think his acting actually got worse as he aged. He's very good in
Stand By Me, and less so each year after that. Especially average as an adult.
The First Duty, even though he's just okay in it.