When it comes to Rogue-likes everyone has their favorites. Some like the streamlined Nethack, others prefer the slightly more refined Angband, and others enjoy things even further out on the fringe. My randomly constructed poison of choice is Ancient Domains of Mystery because I enjoy the everything and a few kitchen sinks approach of the game which somehow at the same time never really feels overwhelming.
Perhaps I should qualify that a bit: it never feels overwhelming for a Rogue-like. Rogue for those of you who don't play games that use ASCII characters in the place of graphics is one of the earliest computer RPG's. Essentially it is a randomly built dungeon that the player wanders through beating up bigger and bigger monsters and taking their stuff until the player dies. People have been building on this simple foundation for over twenty-five years adding more and more to the game. From the simple addition of more equipment to additional classes to completely changing the character system, modern Rogue-likes have adapted.
As in most Rogue-likes, you create a character from a Dungeons and Dragons style class system and then toss them down a hole to see if they survive. Odds are they won't. Rogue-likes are unforgiving of mistakes and since you can only save when you quit there's no way to take back a bad move.
The big reason why I like Ancient Domains of Mystery more than others is that the game is very tiered. You can play decently with only knowing a few of the more common keyboard commands. As you start to get deeper into the game, though, you can add your own layers of complexity by learning a few new tricks. You won't be as efficient as you could be at the beginning but you'll be able to jump in and have fun without reading a dry as dust twenty page manual. Also, for a game that uses keyboard commands and ASCII graphics the game has a very elegant interface. Yes, there's up to three functions for every key press but odds are that you won't have to use more than the basic layer as you get used to the game and many functions that are spread out in other Rogue-likes are merged together for simplicity.
The other thing is that there's a lot of depth in Ancient Domains of Mystery. There's twenty classes and ten races to build your character on and there's a lot of variety among them. Very few of them feel redundant, a problem I have with many other games that pile on the options like that. That variety extends to the scope of actions that you may take in the game. There's a lot there to explore.
If you've never played a Rogue-like other than Diablo before then I think Ancient Domains of Mystery is a good place to start. Its simple to get into but rewarding in its scope.