My favorite adventure game? Thats a though one! I'm a huge fan of adventure games, ever since I was a kid. Granted, I've ignored some of the classic ones (specially after getting stuck in some dead-ends in Sierra games...), but I've played most of them, and I don't think its easy to name my ONE favorite.
Instead, I'll name my top five, but in NO specific order:
-Gabriel Knight 2 - the first one is also amazing, but I liked 2 a little bit more.
-I Have no Mouth and I must Scream - A kinda dark one, but with an amazing storyline and fun puzles. I like the fact that each character has a different plot, but with some major conections between them.
-Blackwell Unbound -Not really a classic, but its amazing! While it feels fanmade (well, it DOES use AGS), the storyline is just great, and the jazz song in the end makes me cry.
-Leisure Suit Larry 7 - The entire series is amazing, actually. But the seventh one is the one that made me laugh the most, and thats what counts the most when I play the Larry games. The numerous times Larry comes back to his room naked is one of the reasons I love this game (gee, this will sound a bit strange for people who never played this!
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-The Dig - The Lucas Arts game that most people seem to hate is actually my favorite. The story is just so well written that makes me feel like I'm reading a best seller. And the puzzles are not bad. Well, I hate the one with the collored marbles, but thats the only one.