Personally, I'd recommend Titan Quest with the Immortal Throne expansion, particularly if you're into Greek, Egyptian, and Asian Mythos. (Though mostly Greek.)
Me and my friend have actually been replaying that recently with a new pair of characters, and I gotta say, I love it. Love playing the Corsair. (Defender/Rogue.) That's another great feature about the game, is the multi-classing. At Level 2, you take your first class out of Eight choices, Nine if you have Immortal Throne, then at level 8, or every level there-after, you have the choice of picking up a second class, which gives you a new, class-combo specific title. It may be old, but it's really a well-hidden gem. Super-linear like Diablo ONE. Diablo II had randomly generated maps, something TQ doesn't have, but there's a boon to not having randomly generated maps... You never get 'Act I' syndrome... I know, it seems counter-intuitive... Don't ask me why it's like that, it just is. And yes, I realize the actual dungeons in Diablo I were Randomly Generated too, but the path of the game was extremely linear.
And despite it's age the graphics and visuals are amazing. Reaching the peaks of the mountains in the third act, and looking down only to see fog really gives a sense of "Damn... Better not fall." Looking down on Magea's beach in Act I from the cliff above, and all the baddies are smaller... It's actually a very beautiful game. And you never get the Diablo II-esque 'Act III' syndrome either. "I'm in a jungle-y swamp. I'm in a monastery in a Jungle-y Swamp... I'm in a Sewer in a Jungle-y Swamp... I'm on a boat... In a Jungle-y Swamp." Every individual area in Titan Quest is actually crafted to be visibly different from it's counterparts somehow.
And let's not forget the classes... Really, you'll spend a larger portion of the game wondering what class you should take or what skills you should get than actually playing the damn game. This may seem bad, but it's not. The reason you spend so much time is because just about every freaking skill in the game, and every class in the game, is just plain good. Sure, some classes have a below average skill or two, but I'd say 90-95% of all the skills in the game will actually be useful. And the skill trees are awesome, giving a real sense of progression with every level, not like Diablo where you spend more of your time thinking, "Damn, I wish I was level 24 so I could get that awesome new skill." That's another thing about TQ. Skills aren't determined by levels, per se. There is a minimum level for acquiring the upper-tier skills, but it's a soft-cap, not a hard one like Diablo's. If you wanna rush to the top of the tree, you can just pump all your points at every level into your Mastery. You won't have any skills doing this, but Masteries are your best way of getting extra HP, MP, and stats. You get stat points with every level as well allowing you to further inflate your stats, though it's generally advised to save them, never put any into HP or MP, and only use them when you need to increase your stats in order to use Armor or Weapons. (Masteries are what the classes are called in TQ btw.)
Yes, I'm totally plugging here. It's a great game. It's Diablo II, but done right, and done better. You'll be playing through it every six months. It's a solid buy.