While not the first hack-and-slash action RPG out there, Diablo was arguably the one that popularized the genre. Its blend of intuitive action and RPG elements made it easy to pick up and play while its randomly generated loot, dungeons and quest lines gave it considerable replay value. These, paired with the fact that the game supported free online play over Battle.net, helped make Blizzard a monstrously popular name in PC gaming in the late 90s and early 2000s.
Diablo II improved upon the gameplay in a lot of ways by having more classes and abilities and generally being much faster-paced, but I always loved the feel of the original game; it had a very foreboding and moody atmosphere that its sequels sorely lacked. Nevertheless, while I played Diablo 1 and 2 quite a bit back in the late 90s/early 2000s, I never actually completed the original game. So that's what I plan on doing this time!