There are many Sudoku apps out there, but this one is my favorite.
Electronic Arts (EA Games) is my favorite company for both PC games & now on my iPhone. They have a long list of games available on the iPhone & I have every one of them. This Sudoku game is clean & easy to use, has 4 difficulty levels, 5 if you complete a puzzle & unlock the hardest difficulty, which I know better than to even try. I installed this particular app for our 15-year-old daughter. We like the the graphics, sound effects, ease of use, & that it shows you when you complete a line, box, or get all of a number in the puzzle by briefly highlighting what you've just completed. The cost is $2.99 & I believe it's worth the price.