2. This spot has to go to Team Fortress 2, what can I say? It's the best FPS on the market in my opinion.
3. At this point it becomes hard to decide the next three games were all very close, first up is Guild Wars, this was an excellent ORPG (I refuse to call it an MMORPG as it is much closer in structure to an RPG but online) It managed to replace grinding for equipment and levels with a the ability to create a decent build (Set of skills) for your character, sometimes the PvP was too reliant on ridiculous reaction times for me but it still played well overall.
4. Close behind it is Iji, it's a platformer/shooter that's completely free and equally awesome. The game is fun to play and full of eastereggs, although the main body of the game consists of only ten levels which wont take that long to complete their is so much to do after you complete those, apart from going back and completing it on various different difficulties you get different dialogue and a slightly different ending depending on how many enemies you kill. If you avoid killing anyone then you get another little bonus and there is a hidden level which is practically bursting full of eastereggs.
5. The last game on my list gets in based mainly on potential. It is the very singular Dwarf Fortress, for anyone who hasn't played it Dwarf Fortress is (or aims to be) a fantasy world simulator, it creates a world then runs through about three hundred years of history before plonking you down to either build a giant fortress for the glory of the dwarves or else play as a lone adventurer with no real set goals. The real area in which Df excels is in the details, the games lone programmer

O) doesn't feel that half measures are acceptable and such pretty much everything is modelled to ridiculous degrees of accuracy.
Anyhow, those are my top 5, please feel free to point out exactly why your personal top 5 are so much better.