I have reached the point in my Sudoku journey where a lot of puzzles are not quite hard enough. Sudoktor has fiendish puzzles which are challenging enough for me.
I like that Sudoktor will calculate and continuously update possible candidates for me, I can do this for myself but honestly it's a tedious unchallenging part of solving a puzzle.
One element of Sudoktor which as far as I am aware is unique, is the user interface it uses when entering either your guess or a pencil mark. You select the cell you want to work with and the display pops up a larger square containing the values that you may choose. The really clever part of this is that it represents pencil marks as small digits and guesses as large digits. This feature has once and for all ended the problem of accidentally entering a guess instead of a pencil mark because it is obvious which you are about to do.
I also like very much the ability to import and export puzzles, other apps have this but Sudoktor seems to go one step further and allow many options for import and export format.
The analysis feature is interesting, it's not just on or off, you can select the type(s) of analysis that you want the program to use. I find ths helpful as a learning aid when stuck. The display shows exactly what your next move should be and more importantly shows you why. You can see the explanation on the actual puzzle you are working on.
All in all Sudoktor is a step ahead of any other Sudoku app I have seen.