I'm gonna have to say a lot of Cactus [http://www.cactus-soft.co.nr/]'s games, especially Ad Nauseum 2, are high up on my list.
It's pretty hard to name favorite games though. I mean, it really can depend a lot on my mood. I think someone mentioned Rock Band earlier, and that certainly is my favorite party game. Nothing better than doing a few tunes with some friends.
For solo games, in the realms of creepy games, I'd have to list either Barrow Hill, Scratches: Director's Cut, Yahtzee's Chzo Mythos [http://www.fullyramblomatic.com/games.htm] (aka John DeFoe quadrology), or BioShock. In the realm of less creepy games, the entire Myst series is simply great.
There are some games I enjoy playing with other people watching (or vice versa), and The Curse of Monkey Island and Day of the Tentacle are both my favorites for this play style. The games are fun even to watch.
As for multiplayer games, there actually aren't many I found enjoyable (which is a shame cause I do enjoy multiplayer games). I'd say Super Smash Brothers Melee is my favorite though, since my brother and I engaging in Gentleman Duels between Marth and Roy is such a challenge. Another good one is Jazz Jackrabbit 2, despite being bug-riddled, which combines platforming and shooting really well. Great for co-op or competitive play, that one. For something more arcady, Bombastic is a riot.
MaxTheReaper said:
People. They have infinite replay value, don't break easily, and there are literally billions of them, so if you do break one, mentally or physically, you can just find another.
And the difficulty curve is less of a curve and more of a straight line, because they're just so easy to play with.
Oh yeah, and this. This is true.