9 Digits
By Ivonne Engemann and Marcus Wagner.
I helped the great folks at 9digits.de redesign their Sudoku app, which also supports X-Sudoku, Killer Sudoku, and MathDoku, with hundreds of puzzles.
My favorite things about it are the super smart hint system, which actually helps you improve as a player, and the unique and very effective input system.
You can find more information at their website.