This is a followup to @SorosFootSoldier@hexbear.net 's recent thread for completeness' sake.
I'll state an old classic that is seen as a genre defining game because it is: Myst. Yes, it redefined the genre... in ways I fucking hated and that the adventure game genre took decades to fully recover from. It was a pompous mess in its presentation and was the worst kind of "doing action does vague thing or nothing at all, where is your hint book" puzzle gameplay wrapped in graphical hype which ages pretty poorly as far as appeal qualities go.
So many adventure games tried to be Myst afterward that the sheer budgetary costs and redundancy of the also-rans crashed the adventure game genre for years.
Half-life as a series had the advantage of being an early adopter to a lot of stuff that is now dated. At the time of release it was revolutionary, but by comparison now it probably feels a little mediocre.
Plus it was one of the few games of the time that really understood how to use a good soundtrack to improve the vibe of the game and I still go back and listen to the OST sometimes.