I haven't tried all of them yet, however I really like Sudoku(free). Also, I found that I really did not like ACTSudokuFREE. Hope that helps.Which one is the best out of all the free versions? I've never played this and i wouldn't mind trying.
Enjoy Sudoku Daily
Sudoku Classic - Free!
Sudoku Classic is no longer free since the developer finally released their Sudoku Daily app, where you get one puzzle a day, but basically the same as the full Sudoku Unlimited features. I just tried it and I really like the newspaper skin, especially how the permanent numbers are a real font like a newspaper, and the rest look hand written blue pen, real easy do differ. And they vary hand writing style, so if you entered two 8s, they will look different as if the whole thing was done by hand and not a font.Classic Free, hands down.