Andy Serkis in Andor.

Finally got around to watching Stargate SG-1, so Tobin Bell. Not sure if it’s in the true spirit of the OP as it was before Saw made him famous, but he absolutely stood out from the rest.

Nick Offerman in The Last of Us.

Michael J. Fox on Scrubs. Seeing him perform again a bit later in his career was great and it showed how much he was enjoying the work.

Brad Pitt showing up on FRIENDS while married to Jennifer Anistone was something i remember a lot of people gushing over when it happened.

But the kne performance I'll never forget is Christina Hendricks in Firefly as Mal's wife.

Off the top of my head, I really enjoyed seeing Alexander Siddig (Julian Bashir) show up unexpectedly in the first episode of Foundation! Even if the role was small, it was a welcome surprise and imo he crushed it.

And then we binged all the rest of it in two days. An absolute must-see for any fans that love a good space opera! Can't wait for season 3.

Chiana and Scorpius on Farscape were both intended to be guest characters, each for a single episode, Both instantly became main characters, with the latter replacing the main villain for the series. And then he did it again when they retconned Harvey into existence.

Babylon 5 has a few really good ones,* including a very early role for Bryan Cranston. But Wayne Alexander as the inquisitor is particularly notable. There's layers to the character and the performance really rises to meet the challenge. He perfectly balances a character that is cold, heartless and monstrous while having a surprising amount of humanity. In his performance you can see the idealist and the cynic, the zealot and the apathetic, cruel but without malice, a man who is totally devoted to his his mission who would like nothing more than to fail. His last line is understated and it should seem like an afterthought but it's only because of the performance and the writing that it can work so well because we're so invested in this character, and suddenly all the pieces fall into place.

* Walter Koenig as Bester needs to be mentioned but I believe he was always meant to be a recurring character.

Teri Garr sure was nice in that original series Star Trek episode.

not a cameo, still my favorite

jamie lee curtis in the bear