Just one? That's so hard! How about five?
Grim Fandango (if I did have to chose one it might be this)
Still one of the best, most original narratives in any game - nothing here felt like a rip off from a movie, comic or novel or other better source. Grim Fandango was a true, original creation.
Great characters with actual heart and a theme and story that actually took you somewhere emotionally.
Some of the best voice acting ever, fun puzzles and just a fantastic ending.
As close to perfection as it gets for videogames. You might not like it, but that's just taste. The game itself has no real flaws or issues inherent in its design.
Please get this on PSN or Steam remastered in HD to really let the wonderful art direction shine.
Ico
Just fantastic. Again, it felt wholly it's own creation - a well constructed little story in the mold of fairy tales and fables.
The single best use of feedback ever via rumble, a perfect ending and a perfect journey render Ico in that rare circle of essentially perfect games.
Deus Ex
Yeah it's flawed, the narrative has a few holes and the graphics even for the time were a bit hit and miss.
But such ambition! Even today few games come remotely close to the depth of world and ambition showcased here, nor have remotely as much variety of missions. Almost everything in the world worked and almost every off course action you could take seemed to have been anticipated. You could take a basketball and bounce it off people's heads and they'd respond appropriately!
Deserved winner of massive amounts of awards, but too much for the average gamer at the time (maybe even still) Deus Ex is a monument to potential and aiming way above average.
Half Life 2
FPS Single Player perfection. Today the focus has swung to MP, and most SP campaigns are frankly, awful and only exist to showcase a few setpieces and tick a box that gamers seem to want ticked.
But HL2 predates that and is from a time when the SP was the focus where you'd start before moving on to MP.
It still represents, in flow, level design and construction of the campaign around variety, clear design and a total lack of reliance on other mediums conventions - no cut-scenes here to convey some exposition - the gold standard for FPS SP campaigns.
From urban run & gun to survival horror to vehicle sections to SF thriller the ride flows smoothly and assuredly from the beginning right to the perfectly aligned ending.
Elite
Elite was the title that made me realize, way back in the early days of videogaming, just how far things might go. It gave you an open Universe and let you run with it.
It was an is Elite.
Now, why the hell can't they produce any worthy follow ups with all the tech we have today?