Star Trek

All the series were blessed with some great guest stars.  The best of them made put an indelible stamp on the series they appeared in, or on a key character,  or added a dimension to the Star Trek universe. 

Who were your favorites?  I'll start with Jane Wyatt on TOS.  In one appearance she helped define the character of Spock.

I'll say John de Lancie. Q added something great to Star Trek and introduced the Borg. Also, Whoopi Goldberg(?) as Guinan.

Goodness -- this is a tough one. There are so many.

If we're talking recurring characters, well I really love Garak, but is he a "guest star" ?? hmm...

One that does spring to mind is Louise Fletcher as Kai Winn, who came on to the scene already as a veteran actress, (possibly even a little typecast in villain roles?? ~~ just 'cos she's so good at it.) and just created a magnificently complex character.

For one time appearances, David Ogden Stiers as Dr. Timicen (the scientist who fell in love with Lwaxana Troi.) Such an understated performance, but the episode that featured him was fabulous, and I really did feel for his character in everything that befell him.

But like I said, so many fantastic guest stars that it seems like a gross injustice to have singled out only these two.

Alice Krige as the 'Borg Queen'.

Why?

(That's to all three of the above, btw.)

Whoopi Goldberg-best bartender and advisor

That was Susie Plaxton wasn't it?  Was she also the female Q?  And Dr. Selar?

Yes indeed Servers - and on two occasions no less!!!

Also of course Mark Lenard who has a pivotal role as:

Unnamed Romulan commander (TOS-BOT)

Sarek of Vulcan (TOS-JTB/TNG/ST3&4)

Unnamed Klingon commander (ST-TMP)

Mark Lenard as Sarek, in my opinion.

Correctamundo on the Female Q. She also played an Andorian on Enterprise (whose name escapes me for the moment because I'm going senile in my 30s uh oh!!)

Coolness. I would have said basically the same thing although was curious to get a more detailed "take" on the matter from someone else. Sarek was a great character.

Shran?

(I'm not completely senile! Yet. lol)

We're off to a great start.  I agree with so many of the initial posts - Mark Lenard, Borg Queen, Q, etc.  In particular I agree with ServalanFan about K'Ehlehr - Susie Plaxton, in my opinion helped define Worf.  And I found the chemistry between them the most believable of the several love interests that Star Trek gave to Worf.

I also agree with the Shran vote. I believe that actor played more than one Star Trek role. 

Who would we cite from Voyager?

Jeffrey Combs as Commander Shran

Q in TNG has definitely made a huge impact. Agree on K´Ehleyr as well. But both are "returning" guest stars. If that counts, too, can I mention Lwaxana Troi and Barclay, too?

From TOS, the first to come to mind are Gary Seven and Roberta Lincoln. But here, as well as in TNG, I´m bound to come up with more if I think a little more about it!