Solo with 3x3 jigsaw + killer at unreasonable difficulty is the most fun I've ever had solving sudokus, requiring some really novel logic. In comparison, the normal 3x3 at unreasonable difficulty is really tedious, often devolving into a series of trial and error decisions.
'unruly' and 'signposts' are neat little timewasters, but devolve into a lot of counting at larger sizes..
'flip' with random shapes is a great fidget-style toy
'galaxies' at 'unreasonable difficulty' gets pretty crazy
and plenty more, eg 'solo' provides every version of sudoku that you might want