I've been using Sudoku Classic Free.
My first ever Sudoku was with the Sudoku Free app, but I found it a little difficult for advance levels since you only can enter a single note number per square. Worse, while the notice is italic, it's the same size and color, so it doesn't stand from your normal numbers. The app does have different board color choices, and easy number entering system.
Though, Classic Free has better notes, letting you enter all the numbers in one square. Only downside to Classic is the board is covered by the number pad to enter in numbers. Some hate this. I find it a slight annoyance. I usually know what I want to enter first anyway, and the pad never covers the row or column of the square you have selected, plus it highlights this in red.
This company said Classic will only be free until their Sudoku Daily comes out. Though, an app called Enjoy Sudoku Daily has come out, but totally different looking and isn't listed under the same company, don't know if that's the same app they spoke of.