My favourite has to be "Keen", it's a sudoku-like where a grid has to be filled with no repeated numbers on either columns or rows, and arbitrarily shaped cells must be filled to satisfy an arithmetic constraint like "sums to 7", "the product is 84" or "one divided by the other is 3" (if sized two).
Towers is nice too, similar concept (re repetition), but the constraints are now visibility ranges on the boundaries of the grid, as you put down towers of varying height. I find it more difficult.
Some of the games are more mechanical, where you can mindlessly iterate to a solution step by step. Like "Net" (rotate pipes to connect them all to the center). Towers takes some more guess work, and I find Keen is there in the middle.