Diablo 1 had the best dark story, theme, and music. I think the game was very good and It's two biggest downsides were 1) Length 2) The fact you couldn't run. Do not play that game online as It's hacked to Hell (no pun intended) because you can easily edit the file saved to your computer.
Diablo 2 was the best in the series and the online was superb. Played it on and off for a good 8+ years. The only other PC games that I played that much were Ultima Online and Warcraft 2. Hell Starcraft came close. Blizzard has made some classics until recently IMHO...
... Diablo 3. Now while I still put 300+ hours into Diablo 3 I think it was a pretty poor entry into the series. It's still addictive as hell but the whole game doesn't feel very Diabloish. Despite only allowing 4 players to play in your game the online is still pretty rock solid. The problem is the story is terrible, music is terrible and I didn't care much for the art direction. Also the Real Money Auction House (RMAH) was defenitely a terrible move for gamers but obviously a good one for Blizzard in terms of generation money. Between high sales and $1 made off of every RMAH sale we are atleast guranteed another expansion/sequel as well as tons of cash flow for Blizzard. Again, Diablo 3 is not a bad game at all, infact I would say maybe a 8/10 but that's pretty poor for the series overall. I'd give Diablo 2/Diablo 2 LOD a 9.8. Diablo 1 a 9.4
If your upset like me with the direction Blizzard has went with the series then I'd recommend Torchlight 1 + Torchlight 2. Torchlight 1 is very good though a bit on the short side and has no multiplayer. Torchlight 2 is more expansive and has online muliplayer. You can try the Torchlight 2 demo on Steam free of charge and it lasts a good 3+ hours. I'd give Torchlight 1 a high 8 maybe a 9, while Torchlight 2 I'm unsure what to rate it but It's damn good.