Honestly, there's nothing better than the Cracking the Cryptic app for sudoku, called "Sven's Sudoku Pad." People who love sudoku have congregated around that channel. It's updated regularly and there are a lot of puzzles that can be imported into it. It gives you very helpful pencil mark controls including colors and differently placed marks. There really just hasn't been another app that compares to the level of production for this Sven's.
Also the novel rulesets make it so much more interesting than vanilla sudoku. Oftentimes the puzzles have really abstract/mathematical/beautiful break-ins. Definitely adds a lot of variety and replayability.
