ADOM was my favorite for a very long time. Your characters often live long enough that you can really strategize, plan...and unfortunately, also farm.
Nethack has less farming opportunities 'cause it's faster paced and that makes it pretty awesome, if frustrating as all hell.
I discovered Incursion recently, and I LOVE it. Even if I don't have the time to play it. If you're at all a D&D fan, you'll think it's awesome that just about every obscure rule is intact (if modified), and obscenely easy to use...Oh, you want to charge that monster like you could in D&D? Well just open the combat menu and hit 'charge', and watch your bonuses go up with every square you move. Magic is done pretty intelligently. DIPLOMACY IS USEFUL. RESTING IS IMPORTANT. (was hurt; walked into a horde of bandits, managed to use my Diplomacy skill to not only convince them not to kill me, but to also convince them to take watches with me at night so we could all rest safely!)
Yeah, Incursion takes forever to do chargen. However--if your character dies before hitting dungeon level 5 or so (pretty likely), you can 'reincarnate', which means you start that same character over--level 1, only your starting loot, new dungeon, but you don't have to go through chargen again.
I can't recommend Incursion highly enough.