Re: Trek’s top actors
I've been watching Voyager for the first time and simultaneously doing a partial rewatch of TNG, which I've almost finished (and have just started a partial rewatch of DS9), and I've realized that James Sloyan is awesome.

The man just makes you feel for every character he plays.
Here's an (incomplete, since I've forgotten many episodes and haven't seen much of VOY and almost none of ENT) list of other guest stars I really liked (not counting the recurring characters, which I've defined as anyone who's been in more than 2 or 3 episodes as the same character):
Harris Yulin in DS9 "Duet" - hands down my favorite Trek guest star performance
William Marshall in TOS "The Ultimate Computer"
Mark Lenard as Romulan Commander in TOS "Balance of Terror" and as Sarek in TOS "Journey of Babel" and TNG "Sarek" and "Unification" pt 1
Robert Walker Jr. in TOS "Charlie X"
William Windom in TOS "The Doomsday Machine"
Diana Muldaur in TOS "Is There In Truth No Beauty?"
Joanne Linville in TOS "The Enterprise Incident"
John Anderson in TNG "The Survivors"
David Warner in TNG "The Chain of Command"
Suzie Plakson as K'Ehleyr in TNG "The Emissary" and "Reunion"
Harry Groener in TNG "Tin Man"
Lawrence Pressman as Tekeny Ghemor in DS9 "Second Skin" and "Ties of Blood and Water"
Tony Todd as Kurn in TNG and DS9 and especially as adult Jake Sisko in DS9 "The Visitor"
William Sadler as Luther Sloan in DS9 "Inquisition", "Inter Arma Enim Silent Leges" and "Extreme Measures"
Joel Grey in VOY "Resistance"