Day of the Tentacle. Perfectly captures the height of the genre, or at least its first peak: Mature graphics given the technical constraints, 4th-wall-breaking self-deprecating humor (the game is a joke and it knows). Nothing is serious and you should be able to try anything and everything without any repercussions other than not progressing (if you don't, you might miss out on a joke).

Broken Sword was the second peak, when stories became more serious and played more like an interactive novel. There are still jokes, but the game itself isn't one anymore.