TOME is probably the classic example. Insane amount of content, but also well done and not designed to just kill you instantly.

For an older example, you could try ADOM. It's pretty hard, often unfairly so, but it's probably the most fun of the "old as shit" roguelike catalogue.

I've also been looking into one called Quasimorph, but I haven't actually played it yet for lack of time.