I never liked Grim Fandango despite recognizing its importance. I think it's too weird in a bad way. For me, it lacks the optimistic fantasy of LucasArts previous entries and is just too, well, Grim. I wouldn't call it a masterpiece like, for instance, Monkey Island 2.
It’s a film noir coded game. It does nail that dark aspect in many scenes and does it masterfully. I recently replayed it and it was remarkable how great it still is after decades of gameplay evolution.