must-have is relative, but I have to admit I've never played any of the monkey island games. at one point I was going to, despite the setting (couldn't care less about pirates and such), but lost interest once again before I could start the first one.
there are many more, but the mi series is the one I kinda regret skipping. and the gabriel knight games, though I didn't like the third one's demo (I tried it when the game was released), but the first two looks good. and I'd also like to play all those fmv adventures I didn't have the opportunity yet, not becuase they might be good, but because I love fmv in games
