Hands-down Patrick Stewart. He's exactly the same in every role he ever does. He has no range, other than switching from "normal mode" to "yelling mode" and vice versa. It bewilders me why he's often thought of as a "great actor".
Really. And which roles have you seen him in? Any Shakespeare? Any historicals? (and I do NOT refer to "Excalibur" or "Robin Hood: Men in Tights" -- although he does a credible Sean Connery impression in that one)
If you want to see Patrick Stewart ACT, try watching the middle episodes of the miniseries
I, Claudius. He plays Lucius Aelius Sejanus, the Commander of the Praetorian Guard who, by blackmail, murder, and political maneuvering, nearly managed to get himself declared Emperor Tiberius' heir to the Roman Empire.
Or how about as Henry Grey in
Lady Jane? His part in that movie wasn't large, but he did well as the father of a 16-year-old girl who was used as a political pawn to grab the throne of England away from Mary Tudor.
William Shatner: The obvious choice. Some people still insist he was a great lead. Some say he was brilliant in Star Trek 2. He wasn't.
I disagree. So much of his Star Trek acting was overacted, or off-beat. But he was absolutely spot-on
right in TWOK.
Patrick Stewart: Sad to say, because he is a good actor, and certainly a better lead than Shatner, Brooks, Mulgrew and Archer. But he is as overrated as fuck by some people. For example, the "THERE IS FOUR LIGHTS!!!!" scene. Absolutely cringeworthy. Would never cut it on a real tv show.
Odd. I saw that episode and I could have sworn I was watching a "real" TV show and not an imaginary one. Why do you apparently not consider TNG a "real" TV show?
Brent Spiner... Michael Dorn... Avery Brooks... Nana Visitor
I can't disagree on those. Aside from some decently funny moments, I don't care for these actors. I find them boring. And Nana Visitor's voice is annoying.
Colm Meaney: I used to think he was good, but I rewatched the scene from Paradise Lost where he was playing a Changeling and it was embarrassingly bad. He's good at playing O'Brien but that's it. The fact that he was fired from new sci-fi comedy "Life In Mars" confirms this.
His being fired from one show proves he was good at playing a different role on another show?

I've seen Colm Meaney on other shows, and he was in a miniseries that took place during Canada's colonial period. He was
quite good in that.
Kate Mulgrew: It's like they thought we have a new Trek so let's get any actor even WORSE than Brooks as the lead! They nearly managed it with Mulgrew and her sub soap opera level performances.
Watch many soap operas?

I can think of several
dozen actresses much worse. Or maybe you just don't like Kathryn Hepburn, who Kate Mulgrew happens to resemble in looks and voice.
The only Trek actors who deserve praise are: Levar Burton, Rene Auberjoinois, Armin Shimmerman, Jeri Ryan, Jeffrey Coombs and Andrew Robinson.
I won't disagree that these actors deserve praise (except Coombs -- could never stand him, no matter which alien he played).
I'm casting my vote for Jolene Blalock. It's not that people are in awe of her acting... but I've heard people speak of her as if she could, you know, act.
I first saw her as Medea in the two-part TV movie "Jason and the Argonauts." I kept wishing just this once that TPTB would re-write the story and let Jason end up with the girl who loves him, instead of Blalock's character -- she was Absolutely. Terrible. And her character wasn't very nice, either.
Robert Duncan McNeill did a damn fine job as Tom Paris.
Agreed.
I think you'd have to see the actor in more than one role to really judge. The actress who was 7 of 9 in Voyager played a nother nearly opposite character in Boston PUblic, which I think is a better indication of the actor then just a single role. On the other hand, even in interviews, Nimoy is completely emotionless. He did a movie of the week in which he played the wife of Golda Meir and it was almost exactly the same persona as Spock. If you're the same in every role, you aren't acting -- you're playing yourself while saying lines other people wrote.
Jeri Ryan is an excellent actress. She is a natural impressionist, and quite natural when she does interviews.
Regarding Leonard Nimoy... emotionless? I've seen him do interviews where he was laughing so hard he nearly fell off his chair.
And isn't a part of acting saying lines other people wrote, anyway?
I haven't seen the others in enough other roles to really judge them. Shatner seems pretty good in Boston Legal, and it is a completely different character. So maybe he can act.
Shatner has had a wide variety of roles, but it's just his Star Trek stuff and other roles he's had in the past 10-15 years that people tend to remember. I enjoy Kirk (although I find him to be one of the most parody-able ST characters
ever), I enjoyed him in the TV movies he was in, he was pretty good in "T.J. Hooker" and he's an absolute hoot in "Boston Legal." I have to wonder, though, how many people differentiate between Kirk the character and William Shatner the actor. It's hard to know where to draw the line.
I'm going to say Shatner and Blalock.
I was re-watching some TOS episodes the other day and couldn't stop laughing every time Shatner went into his overly dramatic narration style.
And Blalock, she was just a terrible Vulcan period.
Kim Cattrall and Robin Curtiss were much, much worse.

The actress they should have had was the one who played the Vulcan girl who was taking the Starfleet Entrance Exam at the same time as Wesley. Her role wasn't big, but she nailed it perfectly.