Jennifer Hale (FemShep from Mass Effect, Bastila Shan from Knights of the Old Republic, Dr. Naomi Hunter from Metal Gear Solid and Samus Aran from Metroid, amongst many others)
Other honorable mentions go to:
- David Hayter (Solid Motherfucking Snake (and Naked Snake/Big Boss in MGS3))
- Keith David (Captain Anderson from Mass Effect, the Arbiter from Halo and Sergeant Foley from Modern Warfare 2)
- Brandon Keener (Garrus Vakarian from Mass Effect, as well as the American Sniper from Company of Heroes and Sergeant Frank Craft in the Tales of Valor expansion.
- Jen Taylor (Cortana in Halo and Zoey in Left 4 Dead)
- Lance Henriksen (Admiral Hackett in Mass Effect and General Shepherd in Modern Warfare 2)
[small]He really does have a perfect voice for a gruff military commander type character.[/small]
Also ( partly because I automatically think of their Firefly characters whenever I hear them in a game):
- Nathan Fillion (Gunnery Sergeant Reynolds in Halo 3, and Gunnery Sergeant Buck in Halo ODST)
- Adam Baldwin (Random Marines in Halo 3, Dutch in Halo ODST and Kal'Reegar in Mass Effect 2)