Oooh, definitely these four:
Gothic (roleplay game)
Stronghold (medieval strategy)
Zoo Tycoon,
Roller Coaster Tycoon
They were my first games, when I had my first computer, and I still love them and play them every now and then.
And the Gothic series (I and II ) are still my most favorite games because of their atmosphere and story.
Maybe also Pokemon Gold/Silver, really loved the big map(s) and the new features like day/night and new Pokemon.