ADOM is a classic roguelike, deep, detailed and difficult. It has both ASCII and fully graphical modes available (you can switch on the fly at any time).
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Pros
Pro Really, really deep
The game has been around for a dozen years or so, most of that time went into adding and expanding features. There are lots of ways to play the game, and you can dive into it literally for years and still be surprised by unexplored options.
Pro Surprisingly well balanced
Older roguelikes tend to have all of the things including the kitchen sink, and so does this. But the game feels cohesive and whole, and feels vast and expansive for its kitchen sink design rather than cluttered and messy.
Pro A true classic
It's one of the four games that really defined the genre, and it still is relevant today. A great addition to any roguelike fans' library.
Cons
Con Complex interface
A common complaint is that there are a lot of commands to learn, although the devs have been working hard to simplify and streamline the interface (both graphical and ASCII)
Con Upgraded version only available via Steam
You can't buy the upgraded version, with all the extra features, from the developer or through DRM-Free sites like GOG, Itch.io, or Humble.
Con Generic and plain setting and story
About as generic fantasy as you can get.