Tied for my favourite adventure games of all time are Riven: The Sequel to Myst, and Grim Fandango.
I highly recommend playing through the entire Myst series, it develops quite well and is just a beautiful universe to delve into. The Longest Journey and Syberia series are also really well done and I highly recommend them too. Of course you can't go past things like Day of the Tentacle, Full Throttle, Secret of Monkey Island, Beneath a Steel Sky, Indiana Jones and the Fate of Atlantis, Sam & Max Hit the Road, Atlantis series, The Neverhood, and of course the King's Quest and Quest for Glory series.
As you can probably tell the 'point and click adventure' genre is probably my one of my favourites. I wish it were a financially viable genre for AAA developers to bring back.