Superb, Terracentral. They formed a lot of the basis for Grim Fandango's gameplay.
Monkey Island is seen as the best adventure game series ever made by more people than Grim Fandango is, even though I personally think Grim Fandango has the edge. But with that said, I couldn't possibly choose between them.
Monkey Island is more legendary than Grim Fandango, though. It's a must-play.