Macworld's Dan Frakes took the time to buy each one of the 18 Sudoku applications and determine which were the best of the bunch. Frakes managed to narrow the field to 3 top contenders.
Sudoku Vol. 1 [$5.99, App Store] by Hudson Entertainment was his first pick. Its described as the best Sudoku application for beginners in that it offers a tutorial mode to help guide you through the puzzles. While the game provides helpful tools and customizations, it only offers 50 levels and requires you to solve easier levels before getting to the harder ones.
Big Bang Sudoku [$4.99, App Store] from Freeverse offers a shiny interface and is said to be simpler to us than Hudson's offering. This version also offers over 10,000 levels across four difficulty levels.
As his last pick, Frakes choses EA's Sudoku [$7.99, App Store] which has been updated from the iPod version and delivered to the iPhone. The game offers 10,000 unique levels and beautiful graphics (pictured above). One unique feature to EA's version is "Newspaper mode" which allows you to manually enter and play puzzles from a book or newspaper.
A more in depth look at each version's interface is provide in the Macworld article.