Caleb from Blood, Duke Nukem, Garrett from Thief and SHODAN from System Shock. Good, that covers the basics.
The one that should already have been mentioned:
[youtube]http://www.youtube.com/watch?v=WPUZIagbl94[/youtube]
The whole game is full of awesome voice acting.
One of my personal favourites is the Guardian from the later Ultima games.
[youtube]http://www.youtube.com/watch?v=JfqOZlNxbfI[/youtube]
For me his voice is the archetype for the Evil Villain Voice Of Doom.
One thing the new Bard's Tale got right was the voice-acting, but with veteran actors like Cary Ewles at the lead, it was hard not to. The vid only has the narrator in it, but it's a start.
[youtube]http://www.youtube.com/watch?v=fQKCYC2Iods[/youtube]
It even has songs in it, which isn't that common for a video game. Which reminds me of another game that, at least in part, had a brilliant bit of voice acting in it...and singing.
[youtube]http://www.youtube.com/watch?v=P_To2f8DjC8[/youtube]
Murph is awesome.
Other games I recall having good voice acting were Afterlife (as well as every other LucasArts game) and the first Jagged Alliance game. Surely you remember Ivan?