I enjoy sudoku and it's variants. My favorite kind is hypersudoku, which is like regular sudoku but there are 4 hidden grids and hidden constraints.
Besides the regular 9x9 Sudoku variants include:
Samurai Sudoku, Jigsaw Sudoku, Overlap Sudoku, Tanto Sudoku, Hyper Sudoku, Sudoku-X and Combination Sudoku
And there are lots more:
What versions do you like?