AAA games tend to be full of terrific voice actors, so it's hard to pick one just for her or his performance without considering the character they dubbed. Later Naughty Dog games (Uncharted 1+) have great cast, Mass Effect 1 & 2 are full of skilled actors, The Walking Dead stands or falls on voice-acting a lot.

A special place in my heart has Jennifer Hale. Her perfomance in Mass Effect was absolutely fabulous, so full of emotions or a lack of it if needed and her intonation was spot on all the time. It was actually thanks to her that I started noticing and following voice actors.

Tony Jay, anyone? Planescape Torment, Sacrifice and Legacy of Kain games. Superb voice actor. The rest of the VA in these games are also consistently great.

My top 10 favorite games for voice acting:

1. Full Throttle
2. Planescape: Torment
3. Fallout
4. System Shock 2
5. GTA: Vice City
6. Portal
7. The Longest Journey
8. Uncharted: Drake's Fortune
9. The Last Express
10. Max Payne

How could I forget? James McCaffrey's narrative is epic.

The italian versions of Last of Us and Bioshock Infinite have a very good voice-acting.

Simon Templeman as Kain in the first Legacy of Kain game.
Doing the narration for his doomed character and the world he single handedly creates the amazing gothic horror atmosphere of the game.

Postal

The Legacy of Kain games, particularly the 2 Soul Reaver games. Sacrifice comes a close second . Also, the one in Planescape is pretty good too .

Mass Effect series. All of them have great voice acting, and even the "worst" actors/actresses are merely mediocre.

Bioshock Infinite. Of course, Troy Baker provides excellent voice work, but Courtnee Draper often steals the show. All the other voice work is good, too.

KotOR 1 and 2. A lot of excellent voice work in this one, my favorites being Raphael Sbarge (Carth Onasi) and Courtenay Taylor (Juhani) from KotOR, and Sara Kestelman (Kreia), Kelly Hu (Visas Marr), and Grey DeLisle (Handmaiden) from KotOR 2.

The Thief games and System Shock 2 come to mind. Stephen Russell is a VA with a great range overall, voicing Garrett, Karras and many more characters.

I don't know if it's necessarily the best in anything, but I absolutely loved the Spider-man: Shattered Dimensions voice talent. The game featured four alternate dimension variations of Spider-man that you could play and each one of them were performed by a voice actor who had previously (or currently in one of the examples) voiced Spidey in a cartoon series: Neil Patrick Harris (from '03s Spider-man: The New Animated Adventures), Christopher Daniel Barnes (from the 90's series), Josh Keaton (from 08's Spectacular Spider-man), and my personal favorite, the very underrated Dan Gilvezan (from 80's cartoon Spider-man And His Amazing Friends) who also voice Bumblebee in the original Transformers series.

This is a lot harder question than "the worst one", can't think of the best one right now but there are five that came into my mind without much thinking.

Max Payne - Whole storyline particularly the main character
Warcraft III - Whole storyline, particularly Grom Hellscream-Thrall relationship and Arthas as Death Kinight
Starcraft - Whole storyline, particularly Raynor-Kerrigan relationship
Sid Meier's Civilization V - The narator
System Shock 2 - All characters but of course Shodan especially

Bioshock Infinity. I remember the voice actress for Elizabeth was reduced to tears when she was asked to do an emotional scene. You can watch it here.

Tim Curry was amazing as Stratos in Sacrifice.

Also, Martin Sheen was fantastic in Mass Effect 2. Too bad the rest of the game was awful.

I generally enjoyed the voice acting in both GTA:VC and GTA:SA.

Chris Barrie was excellent as Simon the Sorceror.

I loved the voice acting in Day of the Tentacle. It wasn't particularly high quality, but it fit the game perfectly.

Anything by Ron Pearlman is worth listening to.

Edit: Oh, how could I forget. Ellen McLain did an amazing job as GlaDOS/Caroline in Portal 1/2.