Puzzle-ish games and platformers mostly.
I played through Mirrors Edge for the third time a few days ago. It is one of the greatest games I've played. Portal 1 & 2 were great too, and fall mostly in the same category.
Other than that, the half-life series is amazing.
For some reason I like single-player better than multiplayer in the general case. It's too difficult to find strangers who are both fun to play with, and at the same skill level.