God, I've wanted to do voice acting for the longest time. Working on it, but it's tricky. I can hardly choose the BEST, but I can certainly point to a few favorites.
Stephen Russell as Garret in the Thief series, without a doubt.
Tony Jay. The one role that strikes in my mind was as Captain Saladin from King's Quest VI.
Liam Neeson and Malcom McDowell did excellent work in Fallout 3.
Jennifer Hale and Steve Blum in everything, ever.
Earl Boen, also, for that matter. LeChuck for the win!
These are just a few that are off the top of my head.