Very basic

Basic

Easy

sudokuwiki.org/sudoku.htm?bd=000000000406000201902000374000105000600020000010043000300000080000000459209000600

sudokuwiki.org/sudoku.htm?bd=001006050200080034080300200310000008006000040000000100700250000002704000104009000

Medium

1 to 9 in every box and row that can contain 9 numbers (vertical, horizontal, market diagonal rows).

hard

This is a standard sudoku where you just need to have 1 to 9 in each big box, each vertical row, each horizontal row.

The following hard sudoku also has to have 1 to 9 in the green boxes:

sudokuwiki.org/windoku.htm?bd=000054000000000010006000700690008020300000008080000090005000400060000030000000000

Extreme I

Some of the following puzzle may also be difficult enough to be placed in the extreme category:

https://www.conceptispuzzles.com/index.aspx?uri=info/article/424