Syberia is a gorgeous entry in the genre. This is a bit of a no-brainer, but you have to play the Monkey Island series, especially the last two (Curse Of and Escape From). Then there's always the old classics, Sam and Max Hit the Road, Day of the Tentacle, Full Throttle, etc. Also the Space Quest series is good. The Broken Sword games are generally very well-regarded also, though I have no personal experience with them.