I'd have to agree with with Ian & co. re the earlier Lucasarts games,Sam & Max & DOTT in particular being excellent. (Besides Monkey Islands 1-3).
Though on Amiga I enjoyed the earlier Indy & the Last Crusade. Despite the EGA graphics, it was still speedy enough to be playable.
Of the more "recent" graphic adventures, I enjoyed playing Discworld Noir.
The Leisure Larry Larry Games always often had some funny moments,although they were a bit samey to play.
I'm glad someone else mentioned the Broken Sword games. They always gave me the impression of games that might've been on the Amiga too had it survived & grown as a CD gaming platform.
But sadly Broken Sword 3,like a lot of other graphic adventure games/projects of that ilk/type got abandoned....
Again the "push" for first-person shooters, & Myst style clones proved the death of gaming-variety......