The best ones:
1. Half-Life series. And I don't have to explain, cause you already know..
2. Worms 3D, Worms Forts. Dunno, why Hunter thinks it doesn't work in 3D. If you master it - it becomes a shitload of fun on internet (a good way to have a rest from CS or HL)
Other games I like:
Delta Force : Black Hawk Down, NFSU series.
Games I dislike:
1. Doom3. It's graphics are awesome, but the story... Bleh.. I mean cmoooon... Meh. :
2. Serious Sam series. Just... Crap. Utterly crap.
3. NFS series. It's old and it sux.
4. Tetris. I hate it cuz I suck at it..
And the ideal game for me would be CS 1.6 and Worms3D combined. I'd love it.