If you asked most people what is the best game Blizzard ever made, they'd probably say World of Warcraft. Well lots of people are idiots. Blizzard has made many great games, but Diablo has to be my favorite of them all.
"But why not Diablo 2?" you ask. Well just hold onto your butt for a minute I will get to that. First I want to get all lame and nostalgic and say they really don't make games like this any more. Even the upcoming (Sometime this century) Diablo 3 looks lame and childish in comparison to the first Diablo game. This was a game that was scary. When I was younger, even the opening movie made me piss in my undies.
It also came with one of the coolest manuals I can remember. It was full of lots of drawings and backstory about the game, which you never see these days. The manual was actually better than many fantasy books I have read. It did a good job of letting you know what was going on in the world and describing some of the horrors that awaited you. And what horrors they were. To anyone who has played this game, just mention "The Butcher" to them and watch them curl into a fetal position.
That's a big part of why I love the game so much. It had such an incredible atmosphere. So dark and brooding, you were always afraid of what was hiding just out of range of your torch. The music was excellent and added a lot to it. Overall it was just such a cohesive experience. Everything came together perfectly to create a game that was very memorable and sticks with you for a long time. To me that is what makes an excellent game.
And that's also why I don't like Diablo 2 nearly as much. It expanded and improved on pretty much everything from a gameplay standpoint, but in doing so they lost the magic of the first. Too many well-lit outdoor environments, recruiting allies, and a less focused story diluted the experience.
Even though the gameplay doesn't hold up quite as well, for it's time it was incredible. It was the first game I had ever played with randomized dungeons and loot. The sheer variety of weapons and armor was awesome and kept me hooked for months. The three classes were pretty basic; Warrior, Rogue, Sorcerer. But they got the job done without any overlap. Finding scrolls, potions, gear and books was super addicting and paved the way for much of the loot whoring you see in MMO's today. I'd say even games like Borderlands owe some credit to Diablo.
If Diablo had a true spiritual successor though, I'd have to say it's Demon's Souls. When describing it to my brother, the first thing I thought of was "It's like Diablo...except the gameplay is completely different". It's got that same dark, mature fantasy feel. Where you are actually scared to face these demons. But you know you have to, because the townspeople are counting on you! And there is something even more important at stake, AWESOME FUCKING LOOT!!