There are a few voice actors who have defined a game or a series:
Nolan North in just about anything (Nathan Drake, Deadpool, etc.)
David Hayter as Solid Snake
Jennifer Hale in just about anything, but especially for the female Sheppard (Mass Effect)
Ali Hillis as Lightning (Final Fantasy 13)
Rob Clotworthy as Jim Raynor, Jim Harper as Arcturus Mengsk (Starcraft)
Debatable whether you could lump Cammy Luddington (Lara Croft in the new Tomb Raider) in that group as well.
This post was edited on 4/28 at 4:13 pm