Are you saying games in general, PC and console? Or PC only?
If irregardless of platform (side note: I got tired of typing halfway down so I didn't do full reasons why):
World of Warcraft: Set the MMO standard that has remained for nearly a decade (10 yr anniversary this Nov)
Half Life 2: Amazing story line, amazing gameplay, amazing graphics for PC gaming when it was released in 2004, and the graphics still hold up pretty well. Most importantly: It was the first single-player game that required online product activation (DRM) through Steam. Arguably, it MADE Steam the biggest and most popular online gaming retail marketplace.
Minecraft: Has proven that graphics alone, nor an awesome storyline, means success or failure for video games. It's simple gameplay and ability to evolve into a variety of different modes, especially for PC gamers, has shown that imagination and creativity is all it takes for a game to be fun.
Halo (FPS): Was the first FPS hit for the Xbox, and has become a gaming legacy for the Xbox 360, and most likely for the Xbox One. Has spawned the hilarious machinima series of Red vs Blue; and pretty much replaced Goldeneye as the number one multiplayer game that millions of gamers can cluster around a TV to play and talk smack at each other.
Gears of War (TPS): Not as huge as Halo in terms of game-changers, but it created a popular visual of gaming of playing from the third person perspective, over the shoulder. The finely tune mesh of gaming mechanics like reload, melee, and taking cover, combined with an amazing story line and fluid gameplay allows this game to sit shotgun, if not in the driver seat, next to Halo.
Warcraft 3: RTS
GTA III: Open World, Action adventure game for the PS2. This game is what made the GTA series so popular, with its use of a 3D game engine, with its open-ended concept of exploring the world and doing what they want. In addition, it's violent and sexual content has also spewed a series of lawsuits against gaming for excessive violence (mostly by ex-lawyer Jack Thompson), and most likely will continue to be cited as evidence for future lawsuits or laws against M/MA games.
Angry Birds: Mobile gaming arguably took a huge leap forward with this blockbuster hit.
The Sims: Will Wright is a genius, who took a concept that wouldn't become popular until Steve Jobs unveiled the first iPod. Ignoring all the then-current standards of gaming (RTS, FPS, Action-Adventure, etc.) he created a life-simulation game where players could create alternative versions of themselves(or anyone) called Sims, and help them live virtual live on their screens. Through no definite explanation of how this concept could work, it became the best selling PC game in history in two years after it's release, with more than 6.3 million copies sold. You could almost say that the Sims has more sequels and expansion packs than Everquest and Everquest 2 have (not really, EQ alone had 19 expansions versus the 7 for Sims)
Farmville (or one of those other mega Facebook games)
Argument: I don't see how DOD: Source and Last of Us would be considered "Game-changers." Namely because Last of Us was just released, and there are other amazing, drama-inspiring games that were released before it. Example: The episodic game series of Walking Dead comes to mind of being an interactive game that inspires a huge emotional rollercoaster as it propels you through the story. In regards to DOD: Source, my argument is that the original DOD was released in 2000, so that would take precedence. It's gameplay modes were in place before Medal of Honor: Allied Assault and Call of Duty were released for the PC in 2003.
I'm pretty sure I'm missing a Nintendo-related game, related to Mario.