I know I'm not eligible for the contest, but after reading several great entries, I just had to write up my own. It's a very very difficult decision, but I have to go with Dungeon Siege. Why?
1. The Farmer. Every story needs a protagonist. The problem with many stories, is the protagonist's personality gets in the way. In a game, I am the protagonist, so I want to play as myself, not some schmuck a writer spent hours developing. This is why Dungeon Siege is so engaging. The farmer you start out as is basically a blank slate to which you can do as you please. While there aren't a whole lot of personalization options up front, you can really mold your hero into someone you can call your own. Of course, this is even better when you connect with your buddies and see what they bring to the table. The co-op really shines when friends can build their characters up to compliment each others' skills and spells. These days, games have far more customization options, but at the time, Dungeon Siege was above average in character customization and development.
2. The Kingdom of Ehb. The setting of a game is just as important as the characters. While the campaign is relatively linear, the world itself is open. There's a lot to explore on your own and Ehb is absolutely beautiful (at least compared to other games at the time). The story, quests, dungeons, castles, woods, deserts, and other regions are all very well crafted. It all makes for a very enjoyable adventure.
3. The Loot. One of the coolest aspects of Dungeon Siege is the loot. The game took cues from Diablo and makes it rain loot. The weapon and armor possibilities are near endless so it's always interesting to see what you and your buddies find. Of course, part of playing co-op is sharing and trading that loot, so this sort of behavior really enhances the fun.
I spent countless hours playing Dungeon Siege with friends, room mates, and brothers when it was originally released. It's a great game to just pick up and play. The controls, spells, skills, and navigation are all simple but still rewarding. There's no skill point nonsense to worry about. If you like ranged attacks, just use them and that's how you advance.
Honorable mentions: Halo (series), Gears of War (series), Left 4 Dead (series), Lord of the Rings Online, Guild Wars, SPAQ2 (Single Player Action Quake 2), Fire King, Super Mario Bros.
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