Definitely Mr Hamill :)

Startopia also had brilliant VO. I can't remember the name of the actor, but I think he sadly died shortly afterwards.

You forgot the iconic "NOOOoooo...." ;)

Individual voice actors rarely stick out for me. But I do notice when the voice acting in a game is exceptionally good (Vampire the Masquerade: Bloodlines), disappointingly mediocre (Oblivion) or really bad (at least 50% of German dubs).

She's good in many things but her ME1/2 voice acting... it's not that good (sorry femsheppers)

My top favorite has to be Jim Cumming, simply by virtue of "Minsc"... Best NPC EVARRRR!!

EDIT.. Jennifer Hale always struck me as quite generic.. I'd never say anything she's done has particularly stuck out from the pack.

Mark Hamill, Keith David, Jim Cummings, Tony Jay, Stephen Russell...

Ridings has been in a lot of games lately. It's not confirmed by I suspect that he voiced the Commandant in Fable 2 and the Crawler in Fallout 3 (two awesome voices).

Here's a man who only made a single video game voice, but ho boy he made it count: British actor Joss Ackland. He voiced Pieter Van Eckhardt in Tomb Raider: Angel of Darkness (not a very good game, but the voice acting was awesome). I wish he was in more games. Just listen: http://www.youtube.com/watch?v=pFUDFwd--y8.

Martin Sheen in the Mass Effect games. the ME2 trailer featuring the Illusive Man would have been enough to sell me, if i wasn't already going to pick up the game anyway.

also: Keith David, Liam Neeson, Mark Hamill, David Hayter, Ron Perlman and whoever is voicing Captain Price in the Modern Warfare trilogy.

Of course Minsc (as others said -- I saw a cartoon once where he was voicing a character. It was awesome.)

The guy from Batman Beyond did the voice for the main character in Advent Rising. He does exceptional work IMO.

And Mark Hamill as others said. He's got a very natural talent. If you've never read how he got into it, he asked to read for the part of the Joker for Batman TAS. Since he was somehow a friend or something, the director let him read first just to get it over with. But he was shocked and amazed at how good he was.

Later, they realized that he also had a rare gift. Usually, voice actors voice their part and then animation is set to the audio. But occasionally, you'll see a scene where it's all messed up. Those are the parts that were later edited and the animation is made and then an audio track has to be added to the video. This is extremely hard to do for voice actors, but Mark Hamill had a rare ability to do it spot on regularly. Hence, you never see a bad joker scene. (my source was a big Batman comicy encyclopediacy biographicy thing I read)

http://www.amazon.com/gp/product/006107327X/ref=as_li_ss_tl?ie=UTF8&tag=legeofleef-20&linkCode=as2&camp=1789&creative=390957&creativeASIN=006107327X
(ref link included)

Ron Pearlman,Mark Hamill,Jim Cummings and David Warner

Liam O'brien does such a fantastic job voicing nerdy/fabulous/psychotic/some combination of the above.

"ALWAYS WITH THE SCREECH OF INSECTS BUZZING IN YOUR SKULL, ANTS GNAWING AT YOUR EYES, FOREVER AND EVER!"

I've always liked Tim Curry's voice acting in Gabriel Knight 1 and 3. It was a damn shame GK2 dropped voice actors, as the dude playing Gabriel is horrible.

Also Dominc Armato as Guybrush is good IMO.

The voice acting in V:TMB was top notch.

Games in GOG with decent VA (that I've played):
Beyond Good & Evil
Deus Ex
Fallout
Fallout 2
Planescape: Torment

What more?

Ehh... what?