There are some many version with slight variation that it's hard to single out just one. Each game fits a different person. I like Sudoku Unlimited

been through a lot..... personally have stuck to enjoy sudoku - probably my favorite with daily multiple puzzles plus a lot of others. And Sudoku 101, and its bigger brother Sudoku 401

I recommend Suduko unlimited, but it's also the only one I have tried, but for the price it does the job perfectly...

Sudoku Unlimited has a very polished interface and it never crashes. I recommend it. But then what do I know; I'm clinically insane.

http://appshopper.com/games/catdoku Cat Sudoku - you read it right, it is a Sudoku game based on images of cats. Why, you ask? This is a game that marries two of my wife's favorite things - Cats and Sudoku. 20,000 solvable puzzles that are sure to keep you entertained for many hundreds of hours. You will be hard pressed to find a Sudoku spinoff that is more challenging than this game. It takes away all of the familiar numbers and forces you to embark into real pattern recognition. This game is excellent training for your logical reasoning/deduction skills, along with general shape/pattern recognition. In addition to the twist on the classical Sudoku puzzle, this game features the following enhancements: 1) Online scorekeeping and ranking (optional) 2) Cute kitty sounds

i personally love enjoy sudoku, and also 401 sudoku but thats expensive there is a lite version called 101 sudoku. Hate all flashy, long loading time sudokus - here looking at you EA

Sudoku Grab. Use the camera to capture puzzles from magazines and books etc. Handy for competitions as it can solve boards for you