Diablo is one of the best PC games made, and you have no excuse for not having played it.
Diablo, though outdated by today's standards most definitely holds up well and it's easy to see why this game started a franchise that is working on a wildly rumored second sequel, the first having done quite well for itself.
It features a hack and slash style of combat, simply running around the room clicking one of two mouse buttons on the enemy monster and hoping that they drop before you do. The ability to assign one move to one mouse button and another move to the second really keeps this game simple, yet entertaining. As a warrior you can go through with just your basic physical attack, or you can choose to play the wizard and force yourself to learn a few hotkeys so as to fully showcase your utility as a magic-caster.
The main hub for the game is a small town called Tristram, with about six active inhabitants, but it still manages to feel lively enough to keep you entertained. These villagers give you quests asking you to descend into the monastery to either do something beneficial to the village, grab an ancient relic, and a few other clichéd reasons. However a cool feature means that out of the pool of available quests you'll only see X amount on each play through. This inspires the gamer to replay the game a second and possibly third time, not only to experience the other classes but also to fully experience the game itself.
The audio and visuals of the game truly add a lot to the experience and bring the player into this war torn tragedy ridden world. The feel of the game while a little dark at times, is fitting and you'll never feel really out of place or ripped from the atmosphere the game strives to create and maintain.
Diablo is a classic, and anybody who owns a PC should have at least given it a try by this point. If for some reason you haven't I'd highly recommend you do so now.