Walter Koenig was a great Chekov, but his role as Bester the PsyCop from Babylon 5 was a more nuanced role that gave him way more to do.

For me Colm Meaney wins this hands down. Good in Star Trek but great in other things.

Layer Cake, It's always sunny in Philadelphia and Alan Partridge: Alpha Papa are the 3 that come to mind first.

Karl Urban was an okay Dr. McCoy, but the 157 different roles he played on Xena Warrior Princess was where he really shined the brightest.

Bakula on "Quantum Leap"

Eve Harlow on "Agents of SHIELD"

Definitely Scott Bakula.

At the same time, I'm getting the feeling they weren't given the greatest scripts or directors to work with on Enterprise.

I was excited to see where Archer and Malcom would go with their characters, but now they've been distilled too far into an angry little men.

I get the reasoning, but come on. Malcom's a complete wanker past season 3, and Archer's face never changes beyond "It's all about the mission!"

Conversely, I believe Robert Picardo nails his hammy characterization of The Doctor perfectly.

Andreas Katsulas had a recurring role as Commander Tomalak on TNG, but he absolutely shone as G’Kar on Babylon 5, especially in later seasons.

Jeff Combs was excellent in "The Frighteners"

Whoopi Goldberg, Sister Act II

It would be easy to just gesture vaguely at all of Sir Patrick Stewart's career. But your question was about better performances than in Trek, and I actually think he did an excellent Picard during the original TNG run. Not quite a hot take, I know.

However, for a double off-Trek billing, I suggest Green room (2015) where he plays against Anton Yelchin among others. It's a pretty taut little thriller about a punk band on the road that realise to their horror that their gig for the night is at a neonazi club out in the sticks. It goes downhill from there.

Benedict Cumberbatch, Sherlock

Mulgrew. Flemeth. (Kidding).

Levar Burton will always be reading rainbow guy, no matter what he does.

Zachary Quinto. Silar. One of the better villains, until the writers went on strike and the show fell off the rails. Granted, this likely just marks the man’s range as an actor.

John Cho. The Rise and Fall of Sanctuary Moon.

Koenig. Bester.

You could argue that everything Stewart does is equally well done. Scrooge. X. Gurney.

Karl Urban, Dredd

Anthony Montgomery (Travis Mayweather) as Postmaster P in Leprechaun in the Hood

Michelle Yeoh, Supercop

There's a an admiral in Star Trek Beyond that looked familiar, I had to look it up and it's Shohreh Aghdashloo. A forgettable role, only noteworthy because I recognized the actor.

She plays Avasarala in The Expanse. Total powerhouse performance.

I think Patrick Stewart was pretty great in that episode of Extras.

Armin Shimmerman was just as good as Principal Snyder as he was as Quark.

I met him at a convention once and got him sign a letter as Principal Snyder, suspending my friend (who's a huge Buffy fan) from Sunnydale High and threatening to expel her. He read it and thought it was hilarious.

Whoopi Goldberg for The Color Purple.

Michelle Yeoh in Everything Everywhere All at Once

Louise Fletcher in One Flew Over the Cuckoo's Nest

Brent Spiner was brilliant in Master of Disguise, a phenomenal movie

Damn, nearly did not recognize Keating, his performance in that gif gives heavy David Tennant to me.

curiously, the podcast "the incomparable" made an episode exactly about this topic (more specifically, performances before their trek appearances ) https://www.theincomparable.com/theincomparable/823/