PC title Dwarf Fortress may well be one of the ugliest video games of all time. It does make for one hell of a lovely picture book, though.
Don't sell the game short on its visual shortcomings, though; it's a deep game, and a thriving community has sprung up around it, at its lightest serving as the glue that holds an otherwise primitive game world together — at its densest coming up with stuff like this: Matul Remrit, the illustrated tale of one particular game of Dwarf Fortress.
Of all the things I love about video games, this is thing I love most. Games that set their claws so deeply inside someone that they don't just play it, they live it, coming up with stuff like this in their spare time.