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