Haven't played enough games to have a top ten list, but of what I've played, I would unhesitatingly recommend:
1. Riven -- best of the first-person, "plop you in a strange place and make you figure out what the hell's going on" games. My favorite adventure game to date.
2. Amber: Journeys Beyond -- the only game I've ever played that actually scared me.
3. Last Express -- not a game I really love, but I think anyone interested in adventure games should try this one out. Played completely in real time, it is definitely a unique experience.
I would also recommend, with reservations:
1. Myst -- started me on the genre, so that should say something for it, I guess.
2. Grim Fandango -- the best written, best voice-acted, best thought-out game I ever played, but weakened by a really crummy interface. I consider this a "flawed classic".
3. 7th Guest -- basically a puzzle game in adventure fancy pants, but a good puzzle game.
4. Outcast -- an action adventure game that got me started on THAT genre.