Ah, look at everyone recommending action roguelikes. The Berlin Interpretation is dead, long live the Berlin Interpretation! I’ll happily n’th Caves of Qud and Cogmind as amazing turn-based traditional roguelikes, and I’ll add to that pile the following lesser-known gems:

• Dungeonmans: Very much a no-frills traditional roguelike but also a very good one and probably the closest thing out there to “DCSS but better”.
• Tangledeep: Borrowing more from the Japanese side of the genre, with things like pets and item dungeons and sharply limited healing.
• DoomRL/Jupiter Hell: This is what it sounds like, a turn-based top-down version of Doom where cover and movement are everything. DoomRL is the original free version, while Jupiter Hell is the souped-up Steam version stripped of all trademarks.
• Rift Wizard: This one is weird but amazing - you can only attack via magic, you have a limited number of casts of all your spells, and you need to clear an entire level before advancing. But you have a mostly-free choice of new spells each level, and the goal is to put together something hilariously broken before the game outscales you.

Some other notable traditional roguelikes which I think are less good than any of the six above but still worth playing, are:

• Angband: A truly ancient free game whose roots go back to the mainframe days. Still has living variants in addition to vanilla, of which IMO the best are Sil and FrogComPosBand.
• Nethack: Another truly ancient free game from the mainframe days, this one was really intended to be a puzzle an entire university would work together to solve. If you try it today, expect to need spoilers.
• ADOM: The last of the ancient free trifecta, this is less arcane and more story-focused than Nethack but has some truly awful dick moves. Spoilers are an absolute must. Sort of like a proto-Qud. The original is free, but there’s an enhanced tiles version on Steam as well.
• Golden Krone Hotel: A more modern game where you flip between human and vampire.
• Sproggiwood: A highly streamlined traditional roguelike where a given dungeon run will last less than an hour, but there’s metaprogression between dungeons.
• Brogue: A free fantasy roguelike that, like Cogmind, completely eschews experience points.
• Cataclysm: Dark Days Ahead: A free roguelike immersive sim set in the post-apocalypse, complete with zombie hordes.
• Hydra Slayer: A math roguelike. You can only kill a hydra by cutting off all its heads, and if you don’t cut off all of them then some number grow back. Your weapons do things like halving the number of heads, or cutting off exactly three heads (doing nothing if there are fewer than three).
• HyperRogue: The hyper stands for hyperbolic geometry.