I find it a bit difficult to judge an actor's performance, since I'm no actress I can't be sure what should be attributed to writing and what part of a performance is an acting choice. An actor might be pretty much just himself and still fit a role nicely.
That said I'll give it a try.
TOS: I only watched maybe half a dozen of episodes, so I can't really say. But given what an icon Spock became and considering how he was the first to play and shape the role of a Vulcan I'll go with Nimoy.
TNG: Both Stewart and Spiner strike me as fairly good actors.
DS9: I'd say Auberjonois, since he made Odo such a complex character whose vulnerability I always bought.
Voy: Picardo. I doubt the doctor would have had half the screen time if he had been portayed by a lesser actor. I like Ryan as well, but I think that she didn't always play Seven the way I believe she should have been played. Russ on the other hand always nailed the Vulcan perfectly, IMO.
ENT: I can't really comment. No one stood out to me.