Ultima III and IV.

PC only:

Oldies:

Tyrian: By far, my all-time favourite shoot'em up. The plot is detailed and rich, there's TONS of weapons, ships and secrets. Fun to replay endlessly.

Wolfenstein 3D: Being the grand-grandfather of FPS (although I"d attribute modern FPS lineage to Doom), simple and precise mechanics and a brisk pacing makes it enjoyable to anyone without any FPS experience.

Constructor: Even though it's buggy (you can't finish some missions on the 1000-year mode) it's a very fun and challenging game. Even nowadays its graphics are quite OK since they went with a cartoonish style. Too bad the sequel (Street Wars) wasn't par (heck, not even 10%) with the original.

Doom/Doom 2: Fast-paced gory-action that never gets old.

Mech Warrior 2: Fighting 50-ton iron hulks done the right way. Graphics are a bit simple nowadays, but the combat is great.

Twinsen's Odyssey (Little Big Adventure 2): An incredibly detailed world packed with lots of secrets, enemies, conspiracies

Not-so-oldies:

Jagged Alliance 2: The pinnacle of Turn-based Strategy. Non-linearity is great and the challenge increases as you capture more cities. There's still an active community that released a non-official patch (v1.13) with tons of improvement over the vanilla game.

Half-Life: An immense overthrow at most action-packed FPS. Being slower paced and more story-driven than most titles back then, it was one of the greatest gaming experiences I've had.

Newish:

Modern Warfare/Modern Warfare 2: Game-that-would-pass-for-a-movie done the right way in every aspect. Too bad the series declined sharp after MW2, being MW3 my greatest gaming disappointment ever.

The 7th Guest - Lots of fond memories, one of the first games I played intensively with my dad. Loved the story and George Sanger's soundtrack.

Mechwarrior 2 (original flat-shaded version) - Truly felt like piloting a 'Mech; much more sim-like and immersive than later installments. GREAT atmosphere helped along by the excellent narrative and Jeehun Hwang's fantastic music.

Space Quest IV - Tied with part III. Later on I really appreciated the clever humor, but at the time I took it as a serious sci-fi epic.

Dune 2 (and 2000) - Two words: Frank Klepacki. Doesn't hurt I really like the novel(s) and Lynch movie as well.

TIE Fighter - The best of the Lawrence Holland games. Great sim, great story. The Imperial Navy - Inner Circle dynamic was a fantastic concept.

Dark Forces - The first game that really made me feel I was *in* an actual Star Wars movie.

Half-Life - No further explanation needed 😀

Descent 2 - All the good from D1, but more, better and more cinematic.

Wing Commander 2 - Loved Thrakath / the Kilrathi as scary bad guys with their own evil musical theme.

Total Annihilation - Epic scale, great music (Jeremy Soul's best imho).

Primordia - A relatively recent game by Wadjet Eye / Wormwood Studios. Truly amazing story and setting.

donkey kong64: probably the first game I played with my dad. he completed the hell out of this one.
super smash bros. meylei: first game I played intensively with my brother. numerous unlockables, tite action, and all your favorite nintendo characters duking it out.

Commander Keen 1, Prehistorik 1 and Street Rod 1. I was three, maybe four years old and I was crazy about these!