The game that started it all!
Dungeon crawlers. Action-RPG. Hack-n-Slash. This genre goes by many names these days, but it all started with just one game: Diablo.
This is the game that created an all new and highly addictive genre. It is based around a very simple philosophy: Kill monsters, get loot. But the game is renowned for so much more than that.
With random levels, random items and 3 distinct classes you would think that would be enough for any game back in 1997. Wrong! With Diablo, Battle.net was launched. A free to access and play online network where Diablo players (and soon StarCraft and WarCraft players) could log on and play the game cooperatively together. So, not only did Blizzard set the trend for dungeon crawlers as a genre, but they did so with a groundbreaking online multiplayer scheme, one which still has an active community of Diablo 1 players today!
The game's true gem (and base for replayability) is its randomly generated levels and items. While the levels always look the same, their layout changes each play through. You never know where enemies will come from, which way to go, or what items you will get. This formula has been reproduced time and time again from other game developers - usually landing their titles the moniker of "a Diablo-Clone."
Either way you look at it, this is the game that has literally killed people (do a google search for "diablo addict dead") this is the game that started the highly addictive hack-n-slash game that Microsoft (Dungeon Siege) and Sony (Untold Legends) have tried to replicate.
This is the keystone of the action-RPG lineage.