Leonard Nimoy is a no-brainer for TOS. As for worst, I always found George Takei awkward in his role. Sorry.
TNG had the obvious talent of Patrick Stewart. Brent Spiner was much overrated, but still good of course. I see no clear "worst" here, except Wil Wheaton never thrilled me.
DS9 had many great actors. I suppose, all things considered, it would be between Colm Meaney and René Auberjonois for best. Avery Brooks was powerful, but a little over-the-top sometimes. Worst would have to be Terry Farrell.
VGR on the other hand lacks a clear standout actor. I have to admit I found Robert Picardo grating as time went on. But sure he's good. Altogether I'd say Tim Russ. Unlike what some people have said, Garrett Wang is in no way bad, it's his role that people really dislike. Top dishonors go to Robert Beltran. "Wooden" is his middle name. And Jeri Ryan, interesting as her role was, was way too one note in it. So she gets runner-up.
ENT had two strong co-leads in Connor Trineer and Jolene Blalock. John Billingsley is great in everything he does. And of course Anthony Montgomery didn't have a trailer on the ENT set, but was stored overnight with the other props.