Day of the Tentacle and Monkey Island have to be favourites.
But, we are forgetting all the amazing games from the nineties, anyhow, such as (to name a few):
Star Wars (Jedi Knight, etc.)
Age of Empires
Lands of Lore
Tomb Raider I, II and III
...and other such classics :D
Obviously all of the music was also fantastic, hurrah for early-mid nineties alterna-rock, the ska revival, and of course cheesy pop!