Well, let's see....where to begin? Frances Nguyen as The Dohlman of Elas from TOS 3rd Season episode "Elaan of Troyius".
Yeah, baby! Just something about ass-kicking Asian women in Go-Go outfits and knee-high boots!
Celia Lovsky, who played T'Pau in the Season 1 TOS episode "Amok Time". She was cool; it was the first time they'd shown a Vulcan besides Spock. She was the typically logical and distant Vulcan, but yet projected compassion and sympathy, too. Her Eastern-European accent made the character.
I also like her because knowing her name allowed me to beat the Trivia Master at a Trek convention and win a prize!
That episode also has great personal significance to me, as I said in this article:
https://forums.joeuser.com/Forums.aspx?ForumID=17&AID=98293&cmd=myposts
Leonard Nimoy as you know who in TNG episodes "Unification" 1 & 2.
It was neat seeing him interacting with the new crew.
Paul Winfield as both Captain Dathon, from the TNG Season 5 episode "Darmok", and as Captain Terrel, of the USS Reliant, in "ST2: Wrath of Khan". Winfield was a decent actor.
William Campbell as Squire Trelane in TOS episode "The Squire of Gothos"....he was the ~~original~~ "Q". Think about it.
One "guest appearance" I regret not getting to see was Roger C. Carmel as Harry Mudd, in a planned episode of TNG.
It was written and in pre-production in 1988, I believe, when Carmel suddenly died of a heart attack.
The TNG crew finds (an older, naturally) Harry Mudd frozen, or somehow or other still intact all those years later, and has to contend with him the way Kirk and co. did.
That would have been a cool episode.
PS: The Romulan/Human character of "Sela", Tasha Yar's (Denise Crosby) daughter by a Romulan offical (from a different timeline where Yar went with back the USS Enterprise C, and...oh, forget it; it's complicated for the non-Trekkers) was played by Denise Crosby, as well.