I've only played the 1st one, and I consider it an outstanding game (but beware of the misleadingly boring beginning). Its first sequel -Beyond- is said to be more-of-the-same but slightly less good. The following games seem too different to compare, going 3D and all... They're less diablo and more gothic/elderscrolls...

So, I'd say DD1. But do take a few other opinions in account too...

Sacred Gold; Nox; Heretic Kingdoms; Divine Divinity; Neverwinter Nights; Baldur's Gate (ALL); Icewind Dale (ALL); Maybe you would like to check the *Eschalon Books aswell.

Divine Divinity - Of all the Divinity games, this is the most similar to Diablo and, for many, the best in the series

NoX - My personal favorite, doesn't take itself too seriously.

Torchlight - From the creators of the original Diablo, but it has its own identity and a good mod scene. Bonus points for being able to run on virtually anything.

Torchlight II - Torchlight's sequel (obviously), with online co-op that's missing from the original.

I feel you, so was I (Rift...).

Dungeon Siege (NOT on gOg, sadly). Party-based but I suppose one could solo the game.

Not on GOG for obvious reasons, but Marvel Heroes is worth checking out. It's F2P, so you'll need to either grind, pay, or get lucky if you want more than one hero, but the starting selection has some good choices, and it's a fun game all round. If you really enjoy it, it's worth putting a bit of cash in to get your favourite hero (<3 Squirrel Girl), but even if you only find it mildly amusing, it's a good freebie.

There are a lot of good suggestions here in the thread. If you don't mind some older play-styles, then I also recommend Darkstone. I liked it better than Diablo, but it hasn't aged as nicely as Diablo.

As others said, NOX, Divine Divinity (the first, the 2nd isn't nearly as good), Neverwinter Nights 1 and 2 (1 is my favorite -- 2 has great options but the camera controls wreck that game). Torchlight is okay, but it gets old too quickly for me. You might like it, though.

If you want to get into some really good stories and characters, then the Baldur's Gate series might be your cup of tea. Icewind Dale is also very fun for party-based action RPGing in DnD land.

GOG doesn't have it yet, but Sacred 2 Gold is probably my favorite that's out right now. It's DRM-free on Steam and Humble Bundle often has good deals on it. (GOG has Sacred GOLD (the first one) and it's okay, but I liked 2 a lot more).

Lastly, if you want some multiplayer fun, Paths of Exile is a free-to-play game that is simply spectacular. I think I might like it better as far as mechanics go than all of the other games I've listed.

Of course, there's Diablo III. If you like multiplayer, it's great. You can get into a game fast, loot and gold is shared easily (you only pick up what is yours, can't even see what others pick up). It's very fluid, but it lacks a bit of soul. Adventure mode has been a nice add-on with the expansion.

Good luck finding something you love. And happy gaming!