Very small puzzles are often disappointing, they tend to be either trivial, or perversely difficult; rarely do they prove engaging.
This makes Suko very special. It is the double espresso form of Sudoku, an intense shot of intelligent puzzling in a compact form.
The idea is to place the digits 1-9 in a three-by-three grid in such a way that the sums in overlapping quadrants add up to given numbers. It doesn't sound particularly original or exciting, but it is the introduction of an additional clue layer, in the form of regional sum cages that function rather like the cages in killer sudoku, that makes the puzzle absorbing and successful.