Some of my personal recommendations would be brogue (probably the most beginner friendly), DCSS (friendly, tons of content), ADoM and Cataclysm DDA (a sandbox survival roguelike, quite a unique mix)
Really love that it's FOSS. I've got a bunch of scripts I run to scan the game files to learn things that are hard to get just from the in game menus.
ADoM = Ancient Domains of Mystery
Cataclysm DDA = Cataclysm Dark Days Ahead
I swear I didn't make the first one up ;) Edit: ah well, you got your answer already^^
Still a better name than NetHack (which is not a game about hacking networks... "Net" just refers to collaboration over net to improve the game Hack). There is also BrogueCE ("Brogue Community Edition").
Dungeon Crawl Stone Soup
Ancient Domains of Mystery
Cataclysm Dark Days Ahead