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).
Ranked in these QuestionsQuestion Ranking
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 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.
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)