For TNG:
+1 for Spiner. Versatile for sure, and pretty much carried "Datalore" - one of the worst scribbled-out plots of any show of all time, and yet he's taking it with the sincerity needed to rise above its limitations. Proof that something that's awful on paper can be saved by other facets in the medium in which it's made. I sure wouldn't want to listen to an audio-only copy, however...
(Okay, the end scene is still crap as he uses a contraction in what surely is a blooper reel gag due to his alleged inability to use contractions as a key plot point (d'oh), but even if they left in the cast laughing in the official episode it wouldn't make a difference. The rest of the story is VERY strong, due to him.)
Dwight "Barclay" Schultz as runner-up since he, after doing "The A-Team", pretty much shows what range he's got and it's a lot.
For TOS, it's a very tough call - in part because it's a but less drama and a bit more pulp:
DeForest Kelley if not James Doohan; the latter mostly because of greater involvement in the movies and in a TNG episode, but Kelley is pretty much nonstoppable. Honorable mention to Shatner as well; he knew when to take it with utter sincerity and when to add in a little wit/comedy/camp without breaking character.
For DS9:
Hawk Brooks , Scorpio Robinson, and Cuckoo Fletcher with ease, though "Our Man Bashir" and others pretty much shows everyone's top notch as actors.
For VOY:
Tim Russ, hands down. Mulgrew and Philips are easy runner-ups.,