I've been playing roguelikes for the best part of a decade, though I've only expanded my knowledge of the genre beyond ADOM in the last year, so the bracketed section in the above quote applies to me.
Best roguelike: ADOM (Ancient Domains of Mystery)
Kept my attention for over 5 years. Early game is lethal, very challenging, and fun. Character development is strongly linked to the items you find (which I am beginning to realise is important to me in roguelikes) - the odd game might generate an early pair of seven league boots or Vanquisher, but simply finding farmable herbs in the early game or failing to find an alter can have a huge effect on how they game procedes.
Major problems are the interface and that it can be very intimidating to get into. The list of commands in the manual is massive. I was playing this game for a couple of weeks before I discovered that I could kick things. I didn't get far in those two weeks. There's no mouse support which is pretty standard for roguelikes nowadays.
Most accesible: Powder and Dungeons of Dredmor
Sometimes in Powder, just getting to the second level can feel like a huge achievement. Characters die very easily. Graphical tiles so those who aren't used to ascii art won't be as intimidated. Simple, straight forward commands. The diety system is fun.
Dungeons of Dredmor is available on Steam for about a fiver. A very succesful release last summer. Lots of work has gone into making the game funny - everything has a description. Not just graphical tiles, but animation too. Interesting character build system. At the beginning of the game you pick 7 of the classes from a list of about 40 and go from there. Early game is lethal. Expect to see "Congratulations! You have died!" a lot.
Best looking (and highly recommended): Brogue
Very similar to the original Rogue. Uses ascii art but a lot of work has gone into make the game look good. It is very pretty to look at. It also has a great user interface (even better than Dungeon Crawl Stone Soup). Very easy to get into - the rules are straight forward - but also quite a complex game. There are no character classes. Character development is driven by the items you find. A wizard is a character that happens to find (and keep) a lot of magical staffs.
Recommended: Cataclysm, Dungeon Crawl: Stone Soup, TOME 4
Cataclysm is a post-apocalyptic survival game. To date I've yet to survive longer than an in-game day. Really interesting system for defining how powerful your character is. Rather than being linked to how much you have experience you have accumulated it's linked to your moral. Power is increased by reading a good book, eating fancy meals, smoking cigars, etc. Interface is at the same level as ADOM's, not very advanced.
Dungeon Crawl: Stone Soup already has a thread here.
TOME 4 is being heralded by some as the next major roguelike. It is a massive game. I've only reached the mid game so far and that required over 10 hours of game play. The world it's set in is very compelling - the creater does deserve recognition for creating a fantasy world from scratch. Unfortunately after playing it enough, the early game is very samey, very long, and not hugely threatening. I got to the point where I could be playing for 5 hours before I got to a challenging point (most of that using auto-explore). Whereas other roguelikes feel like you are fighting an entire level at once, TOME 4 has been specifically designed to focus on combat versus individual enemies. There's no food clock or monster respawn meaning that you can rest to full health after every encounter. I hope these issues get addressed because the game is set in a supebly developed world.
Other games sitting on my hard drive at the moment are Doom RL, the roguelike version of Doom, and ADOM 2, which is in the early stages of development.
