My favourites are definitely Nethack and DCSS (aka crawl). I would also like to try Cogmind (but it's unfortunately Windows-only), and maybe Dwarf Fortress. Other noteworthy titles are ADOM and Moria/Angband.
There's a lot going on these days in this genre that kinda departs from some of the core ideas of a roguelike, the term "roguelite" is sometimes used. It's a fun rabbit hole to go down into, lots of really good games.
Be warned, it’s insanely difficult and a massive time suck.