Logic Grid: Sudoku — classic 9x9 Sudoku, no registration required.
4 difficulty levels, daily Sudoku and calendar, autosave, notes, hints, statistics, and achievements. Light and dark themes, 15 languages. Free, with a banner; additional hints are optional via ads. Progress is only on your device.
Classic Sudoku: daily, calendar, hints, themes. Conveniently on your phone.