With nostalgia involved, Doom. Without it, the list gets longer, in no particular order:
- Mass Effect 2: Thoroughly enjoyed every bit of the first playthrough.
- Iji: God DAMN what a story, i damn near got depressed when it ended, and people tell me i haven't seen half of it yet, quite literally.
- Toribash: Takes the record of captivating me the longest, a whopping 3 years before my interest waned considerably.
- Cortex Command: Highest level of moddability i've ever seen. Spent a year or two making mods for it, which also got me into pixel art.
- Dwarf Fortress: THE DETAILS AND COMPLEXITY. Hot damn. Although the learning curve is brutal and the graphics are ASCII, this is the only game i know of where you can, in fact, build a giant dick of solid gold and crush invaders/puppies/troublesome nobles beneath its tremendous girth (compensation lol). Or, as an adventurer, you can take joy in beating people to death with their own severed guts. Or with the corpse of a dragon. Or you can punch out teeth and pelt people with them.
- Street Fighter 4: Best fighting game i've ever played, full stop. Back in ye olde days where other fighters were actually available for PC, i was far too young and sucky to actually enjoy them (yes i did play Mortal Kombat, no i was not good at all).
One Must Fall: 2097 gets an honorable mention though, because you played as GIANT FUCKING ROBOTS TRASHING EACH OTHER. Imagine that kind of awesome as a little kid, and not to mention the horror that is the final boss, Nova. Probably the first time i got to witness just how unfair the final boss can be.