I like
Real mathematics based games where the maths is simple and obvious.
Tetris is such a game - take all the shapes that can be made out of four squares simply connected by their edges, and drop them at random into a grid ten cells wide and 20(ish) high. Of course the choice of the size of the grid was made by someone - and chosen to make the difficulty of the game 'about right'; but the rules are simple and clear enough not to offend one's aesthetic sense.