To be honest, I don't think there are any. If I love an old game, it's because it's good. That's not to say I won't get nostalgic and brush the dust off of a game now and then, it just means that I won't like the game any more than before.
Anyway, games I do feel a sense of nostalgia for:
-Duke Nukem 3D and Mechwarrior 2: These were some of the first PC games I really played, not counting Oregon Trail in like... 2nd grade at school.
-Fallout: first RPG I really liked, and it's still my favorite.
-Command & Conquer: Especially Red Alert. I seriously can't hear "Hell March" without a bit of a smile, as I recall playing Red Alert in my youth.
-Age of Empires: I honestly can't stand this game anymore, not with all the improvements in its sequels that make the interface seem agonizingly inefficient, but I do remember it fondly.
-Halo: Not all that old, either, however this is what got me back into console gaming. I remember playing through it on co-op with my brother, because it belonged to him. Good times!
-Deus Ex: While it's not that old, it's old enough to bring back fuzzy memories of how awesome it was the first time through, and discussions I had about it's message and themes.
There are also some older games I liked, but apparently not enough to remember what they were called.