Cave Johnson.. Love J.K Simmons work overall but Cave was perfect.

Some of my favorites:

• GLaDOS from Portal 1+2: Ellen McClain did such a fantastic job balancing a monotonous robotic tone with human emotions and adding subtle glitches to show not everything was 100% alright. She defined what a female robot voice should sound like for that decade. I saw that Stephen Merchant was mentioned and while his performance was good and Wheatley's character was hilarious, I think McLain's performance as GLaDOS was more original and memorable.
• Narrator from The Stanley Parable: Kevan Brighting was arguably the protagonist of the game and the player was the annoying mob to him. As the only chacter with dialogue in the entire walking sim game, he had to carry almost the entire game's story, personality, and charm all through his voice. And it's honestly so fun how disappointed, annoyed, and exasperated he gets when the player refuses to listen.
• The Joker from Batman Arkham Asylum: This one is cheating cause Mark Hamill has played The Joker in almost every modern animated medium available. Still, there's a reason his version is so iconic. It's just so bittersweet that he refuses to ever voice The Joker again since Kevin Conroy, the VA for his Batman passed away.

Edit: After reviewing my list, it seems like I like offscreen narrators who insult the player. How funny

How has Baldur's Gate 3 not been mentioned in this thread yet? Huge cast and everyone is going for it. Even the narrator. Especially the narrator.

The whole cast of Team Fortress 2.

I haven't played that game in over a decade, yet if I read any of the lines from the game, I'll still hear their voices loud as day. Even just seeing the word "medic" and I hear Demoman screaming it.

Arthur Morgan in Red Dead 2

The narrator in The Stanley Parable made me lose it so many times.

Vaas from Far Cry 3.

This scene is always such a treat: https://youtu.be/rKMMCPeiQoc

Also ~~Alan Tudyk~~ Stephen Merchant in Portal 2.

Edit: Was just recently watching something with Alan in it and got them mixed up 😅

Dionysus in hades. Dude amps me up so well I pick him over boons I like better.

Stephen Merchant and JK Simmons do a great job in Portal 2

One of the best villains I can remember from a game, Handsome Jack in Borderlands 2. Maybe he was just written so well, but he was amazing.

Narrator in Bastion. Just glorious.

The first new Wolfenstein was amazing. They got actual German actors to play the nazis and it made a huge difference.

Leonard Nimoy - Civilization 4

https://www.youtube.com/watch?v=uE7uiKsfpVE

I think Hades is the best overall, which has been mentioned here. Everything from Supergiant Games is great. I've seen Disco Elysium mentioned too, which is also great.

But I think one game that is underappreciated and not mentioned here is Horizon: Zero Dawn. Ashley Burch won several awards for her performance as Aloy. I think part of the reason it gets forgotten is that the facial animations in cutscenes are bad, at least in the main game, which led to people just not liking the cutscenes very much. The DLC's animations are pretty good though.

Troy Baker & Ashley Johnson in Last of Us will stay with me for life.

Recency bias because I'm currently playing it, but Cherami Leigh, Keanu Reeves, and Idris Elba — and a ton of others — in Cyberpunk 2077. If you like sci-fi and fantasy, you're covered. Asuna from Sword Art Online. Neo from The Matrix. And Roland from The Dark Tower. The fact that the latter two actors have their characters modeled after them make it more than just voice acting, though. Johnny Silverhand moves like Keanu Reeves in addition to looking like him. And Solomon Reed moves like Idris Elba. That hulking walk he did in Luther? It's all over the Reed character. As for Cherami Leigh, well, you can make V look like whatever you want. You can even make her male, in whole or in part (trans/non-binary characters are possible, they even have a male voice actor).

I'm seeing a ton of great answers in the comments though. Shout-out to the person who said Portal. Ellen McLain has a role in Cyberpunk as well. Early on, in your garage, your car is struck by a robotaxi. The robotaxi's AI owner invites you to come to their facility for a tour as well as repayment as a way of apology, and he also has a mission for you. This AI has "children" that have been acting crazy (and, one of them hit your car) and he wants you to bring them back in. One of them is voiced by Ellen McLain. In her GLaDOS voice and with lines from Portal. It's amazing. They had to have gotten permission from Valve for that little stunt. If you like Portal, it's probably gonna be one of your favorite parts of Cyberpunk.

Legacy of Kain: Soul Reaver, for the original PlayStation. All these years later it’s still an amazing standout.

The only two I can think of that haven't been mentioned yet, but I feel must:

Disco Elysium

Skywind

The latter is unreleased, but pretty much all the voice acting is done and the work I've heard is impeccable.

I was very impressed by Cherami Leigh in Cyberpunk 2077. She sent shivers down my spine.

Of course there is Roger Craig Smith from AC II, but I also enjoyed Noah Watts (III) and Steven Piovesan (Rogue).

Dominic Armato (Guybrush Threepwood) is an all time favourite of mine.

Bastion, Hades. Basically all supergiantgames.

Whoever did the scientist's "ACK!" sound from Half Life 1.

I don't remember the actors names but here are characters and the games whose voice acting left an impression on me:

• Alistair and Morrigan from Dragon Age Origins
• Both leads from Firewatch
• Lee from Telltale's Walking Dead Season 1
• Most of the main characters from Red Dead Redemption 1 and 2
• The voices from Hellblade Senua's Sacrifice
• Astarion from Baldurs Gate 3 was head and shoulders above the rest, although the narrator was also very good
• Troy Baker knocked it out of the park as Indians Jones in the great circle
• All of the Uncharted games have fantastic voice acting. Nolan North was born to play Nathan Drake
• Similarly, the voice acting in The Last of Us is fantastic
• In the mass effect OT, special callout to Jennifer Hlae and Mark Meer (the latter especially grew into the role in the second and third game) and also to the actors who voices Garrus and Tali.

I quite like the work in Max Payne, whoever did it.

Half Life : Alyx. Everyone is great but I especially enjoyed Rhys Darby as the guy in your headset. Tony Todd as the Vortigaunts was also really good.

Stay a while and listen...

The special zombies in left 4 dead 1 & 2

I know it's super obscure, but there are some YouTube videos compiling each special zombie and it's like god tier sound. I bet they had fun doing it, too.

To this day, I still hype myself up to do weird sounds by listening to those because they're so good.

Homeworld - Few lines. All brilliant. " Detecting no communication from anywhere in the system, not even beacons." - "We are being overwhelmed!"

Ixion - So many lines that would make no sense if they weren't being delivered with conviction and sincerity.

While Assassin's Creed: Odyssey is a bit of a mixed bag overall, Melissanthi Mahut as Kassandra will always be one of my favorites. Also Courtenay Taylor as female player character in Fallout 4, and Ashley Burch as Pavarti in Outer Worlds.

Vampire the masquerade: Bloodlines has amazing voice acting, it's god-tier. Check out the character Fat-Larry😉

I've always loved the narrator of Darkest Dungeon, I just wish i didn't such at the game so bad.

It doesn't get better than JC Denton in Deus Ex.

Fight me.

It's gotta be the Obra Dinn for me, the vocals are such a huge part of the story and dine really well.

Every single line in the Soul Reaver trilogy is a masterpiece.

Here, see for yourself. The intro alone will pull you in and never let go.

Cpt. Janeway as Flemeth in the Dragon Age series (especially Origins).

The narrator in Indika. Or really anybody in Indika. Especially considering the English actors aren't even the original actors.