Best sudoku apps

What sudoku apps are you guys using? I already have: Sudoku.com Killer sudoku Crackingthecryptic
Sudoku + V

I want a app that contains difficult sudokus.

Sudoku.com is very annoying because of the constant ads and because you can only mark 1 cell at a time.

Sudoku + v: costs money and contain many variants but they are still very similar.

Killer sudoku: app made by sudoku.com as well, I loved in the beginning, but the puzzles are waaaay to easy now.

Crackingthecryptic: LOVE, but it costs money and I have finished most of the puzzles already.