Continuing on with a few more...
11. Warcraft 2
Warcraft 2 basically defined the world of RTS games and inspired a world. It came sort of out of nowhere (Warcraft lived in relative obscurity at the time), and just about put PC gaming on the map.
12. Starsiege:Tribes
While it wasn't the first FPS that made use of the Z-Axis, it was by far the first that actually embraced the Z-Axis. And let us not forget skiing and mortar jumping as actual viable battle tactics.
13. Starcraft
Starcraft really isn't much other than Warcraft 2 with a fresh coat of paint, but it deserves mention here. Why? What other game has managed to take nearly a full 1/4 of the earth's population hostage by its gaming prowess? Yeah, I'm looking at you East Asia.
14. Wolfenstein 3D
The granddaddy of shooters, and guess what... it still isn't dead. The new breed of low power devices such as smartphones and tablets are going to go undead nazi zombie hunting forever more.
15. Half-Life 1/2 (All Eps)
Half-Life spawned a shooter woven into a story line that was straight out of the nightmarish hells of government conspiracy and tin-foil hats. The gameplay itself is one thing, but the mastery of the story and how the actual gameplay ties in is unrivaled, even to this day. It's not a short jaunt from here to there blowing alien bits to pieces along the way... it's a thought provoking story that painfully spans chapter after chapter, episode after (unreleased) episode. And it keeps us wanting more.
16. Left 4 Dead
Left 4 Dead makes it up on this list not because it's a quality shooter, but rather because it's a team based shooter with a story. You have to actively keep your team alive, drag them back to dead, and fight off hordes of zombies at a time. It's about as close to a true adventure based FPS there is without turning into a full fledged RPG.
17. Counterstrike
Counterstrike gets the nod for being a team/objective based shooter with some decent realism, but doesn't go over the top to try and shoehorn a story around it. Frags, money, guns. It's the granddaddy and everything else has simply been imitation. Flattering, isn't it?
18. World of Warcraft
This will be arguably my most subjective pick. WoW could easily be considered #1 on the list for all of its merits. My personal distaste for its community is what has it slid down into the tier 2 grouping. For what it is, World of Warcraft is a masterpiece - it set the bar for MMORPGs. It's not the first, but it refined nearly every element of the MMORPG genre into oblivion thanks to Blizzard's exacting standards. Unfortunately, while a masterpiece, the polish soon wears off when you find yourself slogging through piles of shit knee-high, also known as "guilds and world chat", and the relatively closed and repetitive nature of the beast.
19. Grand Theft Auto 3/4
Open sandbox world describes why GTA has to go up here. It's a world alive, and you get to be the bad guy. The story is well laid out and it's a completely "American" game... it's refreshing.
20. Diablo 1/2
Wow, I should have rated this higher, but in retrospect it suffers from replayability issues (both Diablo 1 and 2) where it turns into "gotta catch em all" for runes and weapons and such. Multiplayer was where it was at, since you got to basically play through the story as you would in single player. There was no reason to not play multiplayer. But it got boring fast. It's redeeming quality? A great story, and hardcore mode.
21. Homeworld 1/2
Talk about crafting an epic story... Homeworld gave you the feel of being in a SciFi movie or TV show similar to the likes of Babylon 5 or Stargate... a big, huge universe with not too many friends and no comforts of home. It set the stage for many similar 4x space RTS games to come in its wake, but none have captured the feel of a cinematic style story line the way Homeworld has.
22. Dungeon Keeper
Believe it or not, I've never actually played Dungeon Keeper. I've seen it played by several friends though. That's not to imply I wouldn't want to play it, the concept is unique. You get to be the bad guy. You get to take on the cocky good guys. You get to be the evil overlord. Mas cool. Mas cool.
23. Black & White 1/2
Being god has its perks. Controlling a gigantic beast that oversees your people is also perky. Being able to smack around said gigantic beast with the hand of god is just plain epic.
24. The Sims 1/2/3
The Sims gave casual gamers what they want... an honest to goodness virtual life in which the player controls nearly every aspect of their virtual self. For instance, you want to party, you send your sim to a party. You want to eat, you have your sim fix a snack. You want your sim to wet itself, well... you deny it access to the bathroom. You want to sleep with your neighbor's wife, you court and bed her. You want to get impregnated by an alien, you go gaze at the stars for a while. You want your sim to die a horrible unspeakable death, you devise whatever manner of death you can possibly think of. It's a Maxis game for god's sake... the goal of Maxis's games is to devise such dastardly demises for virtual lives as humanly possible. If there was any other possible description for The Sims, it would be The Seinfeld of games - a game about nothing.
25. Oblivion
What other fantasy medieval adventure in 3D could be played so many different ways and inspired so many different addons? Oblivion took a genre into first-person 3D in a way none of its predecessors could.
26. Team Fortress
Fast paced team based shooter with a dose of comic relief built in. It's the defining game to sit back with a couple friends on a LAN with a few beers and laugh at the bad guys with.
27. Battlefield (All)
I have never, ever played any shooter that ever made me feel like I was in a war until I played the Battlefield games. Particularly Battlefield 2. It's not a tactical shooter in any sense of the word... there's bullets flying everywhere, vehicles trying to run your ass over left and right, fighters dropping bombs on you, the list goes on. Bad Company 2 takes it a notch further with the destructible environments coming down on top of your head. It's gratuitous, like playing a game based on the A*Team... except the bullets don't always miss.
28. Company of Heroes
While the defining moments of RTS's have been defined above, Company of Heroes deserves special mention because it uses the world. A real setting, very accurately representing one of the worst wars in human history in gory cinematic detail. Not only that, but the gameplay world was usable, affording destructible environments and a cover mechanism that even took advantage of the destructed environments. There's nothing more satisfying than torching a building where the enemy squad is holed up in, and watching it come down and flatten them. Nothing.
29. Portal
FPS Puzzle game with a really annoying antagonist that you just want to punch, repeatedly. The gameplay is top notch in a glass half-full sort of way. But what really makes the game is knowing the developer took extra time to make very special attention to the subtle jabs at the player at every juncture along the way.
30. Privateer
Wing Commander defined the genre for space combat sims. Privateer took it one step farther, letting you make your fortune across the stars, and more importantly letting you feel like you were in control by customizing your ship.