Tough call...
Honorable mentions:
-Newer super mario brothers wii: this NSMBW mod is just better than any other Mario game out there. Saying it's great doesn't do it justice.
-Rise of nations: I never really got "into" age of empires. And no matter how I look at it, aside from the RTS mentioned below, it ticks all my boxes
-starcraft 2: no...not THIS rts. While great, incredibly balanced and refined to the extreme, it's not MY greatest RTS.
So...here goes:
10. Torchlight 2: the game I've sunk most hours into on steam except one. It's a bit too mindless, but damn is it fun and addictive like crack.
9. Rock Band 2: while 3 was strictly better, I had the best memories of this one (also with friends). Basic track list was also much better, IMO.
8. Fallout 2: 1 & 2 are still the only turn based RPG's I can really fathom. And 2 was the best of 'em.
7. Zelda: link's awakening: it's actually a tie with oracle of seasons and oracle of ages, but this one got me the best memories.
6. Rollercoaster tycoon: 2 was strictly better, but again: best memories. Never got into 3, unfortunately enough.
5. C&C red alert 2: this is the best RTS ever if you ask me. Wacky but not TOO off the wall. Interesting unti mixture, fun units...things simply couldn't get better (aside from the net code).
4. Anno 2070: I'm hesitant to put such a recent game on this list (I only play it for like a month now), but it clocks in at 70 hours and I have no signs of boredom whatsoever. And at the very least, it's the best city-building game I ever played (though it's not saying much, as I don't play them often).
3. Unreal Tournament: the original. Though 16 years old, it still holds up pretty well today. I played the remake pre-alpha a couple days ago. It's interesting as well. But even then, this is a milestone in gaming history.
2. GTA: San Andreas: while there are plenty of decent sandbox games, this one still captures my heart. (note: I've yet to get 5 at some point).
1. UT2004: the easiest picking. Best game ever. Plenty of great gameplay modes, awesome online community. I've wasted YEARS on this game. Except that it was never wasted but well-spent.