Diablo 3

The Steam version of DS1also doesn't include an ending. Another reason to buy retail.

My favorite isn't on gog yet, but hopefully will be soon enough: Throne of Darkness. A good chunk of the programmers for this game also worked on Diablo, but it easily holds up on its own.

Titan Quest = boring.
Path Of Exile = really boring. Not surprised the game is free...

So if you don't want to pass out from boredom, I suggest The Incredible Adventures Of Van Helsing or Diablo III or Throne Of Darkness or Sacred or Sacred 2.

Path of Exile. It's free, so if is not good, well, you didn't spend money on it.

http://store.steampowered.com/app/238960/

They say Titan Quest. I still have to try it, though.

I would recommend diablo 3 with the expansion to be honest. The game has improved a lot since it initial release and almost everything that was bad about it has been removed or improved. Not saying it's perfect now but it is actually a lot better and they are still improving it.

Also a good choice are the van helsing games and hopefully soon grim dawn.

Path of Exile is as close as you can get to good hack and slash experience and is quite similar to Diablo in a lot of aspects. It is also fun game on it's own.
As was said, it's benefit is that it is good. (you don't have to use Steam as people linked you here, you can install it from developers page and not use Steam at all). On the other hand it is always on game, so you need to stay connected to internet to play it. You can play PoE in party but it is not MMO.
From others Torchlight II is very good. It is much more colorful and less serious game than D 1/2 Torchlight 1 is not bad either but T2 is just more fun in every aspect and T1 can get boring over time.
Ttian Quest is neat game and worth the shot but a lot of its mechanics is not clear enough and I found it lacking and bit unfriendly in some departments. Also if you don't have the correct build later in game and in expansion you will struggle a lot.
Divine Divinity is more story oriented but ti is fun game. In the other half it obviously rushed, though, and doesn't offer much in its strongest departments (quets and story). It's also not so loot heavy as other mentioned game.
Sacred 1 is quite neat game and its main benefit are varying characters to play. I never got deep into it because I had it borrowed but I bought it last week so I can finally get into it.
Dungeon Siege I found quite boring and uninteresting.
I haven't tried Van Helsing yet (I have it though).

+1 to Torchlight.

You don't.

Path of Exile is the most clever hack n' slash since Diablo(s) IMHO. And I mean that's more clever than any Diablo.

Edit : and yes it's free. You can't be wrong with that.

No one mentioned Nox? I like Nox me...

http://www.gog.com/game/nox

I also remember finishing Sanity: Aitken's Artifact which is certainly... different - Diablo in a modern day, superhero setting!

I'll second Din's Curse. It's pretty Diablo-like without being a direct clone. I play quite a bit of Din's Curse.

Titan Quest
Nox
Sacred 1
best i can remember besides diablo 1,2

Torlight and Torlight II.