By series (except Enterprise), in no particular order, and entirely IMO (acting ability being very much in the eye of the beholder):
TOS: DeForest Kelley
TNG: Patrick Stewart, Brent Spiner
DS9: Nana Visitor, Rene Auberjonois, Colm Meaney, Andrew Robinson (practically a regular)
Voyager: Kate Mulgrew, Jeri Ryan, Tim Russ, Robert Picardo, Roxann Dawson
Guest stars: Mark Lenard, Jane Wyatt, Joel Grey, Tricia O'Neil (the Rachel Garrett performance), Jean Simmons and David Ogden Stiers (whom I'm generally indifferent to but thought was brilliant in "Half a Life") are the names that come to mind at the moment.