There are three other roguelikes worth mentioning here (alphabetical order):
A.D.O.M.
A roguelike that has a lot in common with RPG games. It has a story, quests, etc. It has a lot of interesting ideas, depth, and memorable places. It's just fun. It was my first roguelike. It has some cons, too: it's not being developed anymore (bugs are not going to be fixed, etc), story gets annoying after a while because you have to redo quests in almost the same order each time you start anew. It has some almost worthless skills, like bridge building. It's quirky and on many occasions plain unfair, without a chance to escape.
DoomRL:
The simplest of the three, but it's also refreshing. A breath of fresh air in the world of roguelikes. It's quite simple, but deffinitely fun. Not only it's the most notable Sci-fi roguelike around, but also one where ranged weapons (not spells) actually matter. Even better, ranged weapons pack a punch and it makes tactics important.
Linley's Dungeon Crawl (Stone Soup variant)
http://crawl-ref.sourceforge.net
My favourite roguelike. First, it prides itself on being mostly a tactical game and has few quirks. Mechanics are mostly transparent to the player. Spoilers are of little help in Crawl. It's skill-based rather than class-based, meaning you can shape your character in any way you wish. It has a lot of interesting, innovative spells and items. It's technically more advanced than ADOM, has more gizmos, much more varied random levels etc. It has really good interface - for example you can travel between explored levels by just pressing a few keys, auto-explore long, dull corridors until something interesting happens, etc. Gods of Crawl are a lot more interesting than those in Nethack or ADOM. Each of them has its likes and dislikes and rewards (mostly) something different. Crawl has a lot of depth, a lot of variety (many enemies which require unique tactics, for example giant bats which are several times faster than you and use hit&run tactics), and is also quite fair. It rarely has instant deaths which you can't prevent by playing safer. Also, it's almost immune to scumming - there are very few ways to cheat the system, and you can't, for example, stay on the same level to grind forever (Nethack is infamous for this). Crawl has quite high difficulty, and wanting you dead is how it shows its love. Running away is a common survival tactic in Crawl. But, as said before, it rarely kills you without a warning.