Angband is of average complexity but easy for a roguelike. In my opinion it is far and away the best one to start on (okay, maybe ZAngband, but none of the totally whacked-out ones. Angband itself is still fine). It is very survivable because nothing forces you to go deeper into the dungeon; you can tool around in random easier dungeon levels for as long as you like. No pressure.
DoomRL is simple (relatively) but of average difficulty for a roguelike. However, it is also short, and you are likely to win it at least once within your first week (different difficulty settings help).
Nethack is of high complexity and hard for a roguelike. It is also the gold standard, and the deep end of the pool. Jump in. Prepare to die. Think of it as Contra: Losing on the first couple levels is normal. Most players have never come close to winning. Avoid spoilers...for a little while anyway (there's no way in fucking hell that you would learn some of the tricks otherwise).
ADOM is of moderate complexity and average difficulty for a roguelike. It has a lot of gotchas and Guide Dang It moments, though. It is almost as survivable as Angband, which is to say, very survivable: There's not all that much forcing you onward if you don't want to (Or is there?). In my opinion, its magic and archery systems are more fun than average roguelikes. Archers are a true joy to play. Skills are annoying as all fuck, and you practically need to read the Guidebook to get them right (if you don't pick a race/class combo with the Healing skill, or know how to get it early on, you don't have a chance).
Elona is on fucking crack, and the game balance is totally whacked out. You want to avoid gaining levels for all practical purposes, for example. It bends a lot of roguelike molds. I do not recommend it until you are more experienced with roguelikes; otherwise, you will get the wrong ideas.