I've probably wasted too much time and energy on this game, reaching the inhouse league in my country. I just can't believe how fantastic this game is though. The strategy, planning and skill involved makes this game incredibly dynamic. The teamwork element can either have you and your team patting each other on the back for perfectly executed team moves or at each other's throats for incompetence. The graphics and atmosphere of course lack due to the limited WarCraft3 engine, but to be honest in terms of gameplay the engine suits it perfectly. I was almost like it was made for DotA, as opposed to DotA being made as an addon for WC3. All in all I've had some great laughs, fun and frustration with friends and randoms like within this game.
World of Warcraft
This game got me through the honours year of my degree. The social element kept me company after my late hours of studying and insomnia from stress. The competitive element kept me on edge and having fun. Blizzard sticked true to their high reputation in this game, as there is much more to it than just the grinding/addictive gameplay. The atmosphere, sounds and music are wonderful and one can easily immerse oneself in the setting (I often had private roleplaying in my mind when playing). The PvP system is incredibly complex and fun and caters for your every playstyle with battlegrounds, arenas and different classes. PvE can be a little boring if you're grinding, but trying to do a dungeon/raid with friends can be loads of fun if you've never completed it before. All in all though, this game probably takes too much time and attention and I'd rather not play it again.
Blood
I remember seeing Daedalus play this game at his house when Blood just came out. We switched the lights off and it was actually kind of scary for us. I rediscovered Blood when I was in my first year of university and played it quite a lot since then. I don't think I have to comment on the game itself, as I'm sure you all know enough

Dawn of War
The best RTS ever made. It is totally unmatched in atmosphere and sound. The gameplay seems to fit right in with the atmosphere as well. The developers seemed to have mixed their own style of play (seen on Company of Heroes as well) with the tabletop WH2K style. The result is some really epic battles that keep you on the edge of your seat for the whole game. Had tons of fun with Matt specifically on this one. For the Emprah!
Starcraft
This applies to all releases of the game. The game's balance and competitive element is incredible. I had many hours of fun playing this with friends (SC1 and 2). Needless to say the campaigns, music and atmosphere are wonderful as well.
Diablo
I have spent a disgusting amount of time on this game (1 and 2). The atmosphere, music and visuals are incredible if properly appreciated. The gameplay is great, but I still wonder why I keep playing this game over and over. Anyway, I've also had many hours of fun with friends on this one.
Baldur's Gate
Undoubtedly the best RPG of all time (Mainly Shadows of Amn and Throne of Bhaal). This game is incredible in just about every single way. One of the most impressive achievements, which I don't think any other game has achieved ever, is that not one single fight in that game is random or pointless. There is always a reason or story for fighting the enemies you fight. Whether it be some muggers who want your coin, the crazy wizard you fight in the underdark or the giant dragon in his lair, there is dialogue and a reason behind EVERYTHING. I have played this game many, many times and if I had the opportunity I'd play it many times more in the future.
Jedi Knight
I only played the ones from Jedi Knight: Dark Forces II and onwards. I can't think of anything particularly fantastic to say about this game, other than that I really liked the gameplay and played them all many times. There's just something about those lightsaber fights which makes me win, load and replay them many times, savouring each moment.
Deus Ex
I only played through this game twice, but even so I think it is one of the best games I ever played. Story, gameplay, atmosphere, music, etc. They're all perfect and work together to form a masterpiece of a game. I wish I had given this game more attention, but oh well.
I think I'll leave it at that, as I can't decide on the final one. I'll just leave honourable mentions

HONOURABLE MENTIONS
Age of Empires, Red Alert, Worms, STALKER, The Witcher, Unreal, Quake, Left4Dead, Half-Life, Alpha Centauri, Mass Effect, Master of Orion 2.