In no particular order:
Dwarf Fortress
Unlimited Fun, with a capital "F". The same "F" belongs to whatever connotations of HFS you prefer. Not to mention constant development with more to come.
Morrowind
Astounding atmosphere and unending replayability for those who can get over the clunky combat for the sheer joy of exploring a well-crafted world.
Battlefield 2
The most fun I've had in a multiplayer shoot-em-up. Great times with friends, introduction of commanders, awesome expansion, free booster packs post-patch.
Mount & Blade
Emergent gameplay, large-scale battles, best melee combat system I've encountered.
Riven
What I point to during the omnipresent "Are games really art?" debate. A truly fulfilling experience worth the insane puzzles it took to beat.