One of the puzzles that I find particularly satisfying is 3Doku, which takes the usual sudoku rules and expands them to a 4x4 cube, so you have to work out 1-16 on each cube face, each cube row (that wraps around the cube) and each cube column (that also wraps around the cube). The puzzles are simple early in the week and progress in challenge and trickiness until the hard and fiendish ones available on the weekend. I also like it because there are plenty of accessibility options available for the puzzle so that you're not trying quite as hard to have to recognize all the little numbers and figure out where they go and where your next move is coming from.