Top 10. The usual boring stuff that everyone knows and lists already.
Monkey Island Series (First three games).
Full Throttle
The Last Express
Sanitarium
Grim Fandango
Sam & Max Hit the Road
Day of the Tentacle
Gabriel Knight 1
KGB
Bad Mojo (! forgotten gem!)
Everyone knows this stuff already so for those that are interested I'll give an alternate list.. If you're bored these are worth checking out...
Anchorhead (great IF lovecraft game)
Bureau 13 (multiple solutions, C&C)
Maniac Mansion (has C&C! different paths for every combination of characters you choose!!)
Nomad (Starflight but with more and better plot)
A Mind Forever Voyaging (IF science fiction, classic)
It Came from The Desert (parodies old horror flicks, mix of gameplay styles, has C&C!)
Dark Earth (action/adventure)
The Neverhood (clay animated adventure.. excellent)
Little Big Adventure (stylish isometric action/adventure)
Dune (adventure/strategy hybrid.. how often do you see that.. beautiful graphics)
Bioforge (great sf third person adventure/action, somewhat forgotten nowadays)
Discworld Noir (pratchett in noir setting)
The Great Escape (old c64 game.. timed events in prison camp have to be learned by the player and followed correctly, and in between a tunnel must be dug and concealed, and eventually you escape through it)
Waxworks - (horror, first person, nice atmosphere)
Commander Blood (It's shit, but how many games you know with this kind of opening sequence
http://www.youtube.com/watch?v=UCWDNMH36kg )
Grandad - The Quest for the Holey Vest (I'll let the screenshot speak for itself)