The following are difficult sudokus:
https://www.linurs.org/sudoku_solver/index.html?sudoku=89.......6.18.9...5.47...8..8..1..2..4..9..6..5..3..7......46.74..6.23.5.6....248
https://www.linurs.org/sudoku_solver/index.html?sudoku=...6.9..26.3.7...5.1...5.3.436...2.1.9.....7.1.7...569.6.4...5.7...1.3.49..7.3...
https://www.linurs.org/sudoku_solver/index.html?sudoku=.19..2..82.7...14.34......2....4..6....7..9.18.....2...2..73..6.8.2.....4.3.1.7..
https://www.linurs.org/sudoku_solver/index.html?sudoku=...6...5...8.7.3..2...4...91......2..6.5...8..3..9...47...1...6..9.3.5...4...2...
https://www.linurs.org/sudoku_solver/index.html?sudoku=8..6....2.4..5..1....7....3.9...4..62.......87...1..5.3....9....1..8..9.4....2..5
https://www.linurs.org/sudoku_solver/index.html?sudoku=..3425..9.......4..52..9..7......7..34.271.95..1......2..3..97..6.......8..9471..
https://www.linurs.org/sudoku_solver/index.html?sudoku=...9.2..31.9.........4..7..2.8...3.........9...5.6.81..6.......7...14.5.....87...
https://www.linurs.org/sudoku_solver/index.html?sudoku=.....95...3..7...24.6..8...9..7..4...8.....9...2..1..6...3..6.85...4..7...12.....
https://www.linurs.org/sudoku_solver/index.html?sudoku=......42.278....1.1...5..3....4.5.....9...1.....6.7....6..4...3.8....762.52......
https://www.linurs.org/sudoku_solver/index.html?sudoku=.2...69...6..5..324........8..6.2....9..7..8....5.9..4........115..3..4...97...2.
https://www.linurs.org/sudoku_solver/index.html?sudoku=.42...6.716....48..3.6.....9...64......8.7...3..21...4.....1.48..1....23..87...1.
https://www.linurs.org/sudoku_solver/index.html?sudoku=.27.........2.13....6.5.4...5..6..2....3.....9...1..567........1.8.......6.4...89
https://www.linurs.org/sudoku_solver/index.html?sudoku=63..........836............5.49.23....73.49..........7....9.8....851..4...9......
https://www.linurs.org/sudoku_solver/index.html?sudoku=.....93.7.9..3.61......4.....1...4...8...7.694.25.......7..32....8.......1.8...7.
https://www.linurs.org/sudoku_solver/index.html?sudoku=8.7..654.1..8.3762....7.81.....8.1962......85698.....7..4.2...83..4.8..1.8.3..6.4
https://www.linurs.org/sudoku_solver/index.html?sudoku=2...............547.8.4.612..5..9.61...2.5...13.7..9..681.5.2.959...............6
https://www.linurs.org/sudoku_solver/index.html?sudoku=1837..25.2.9..57.85.7.28..3974..2.3.612....75......92.4..1....2..12..3...26..9.8.
https://www.linurs.org/sudoku_solver/index.html?sudoku=.4.....7.2..63...8...2.8.....2...79..1.....4..39...1.....9.1...3...87..4.2.....8.
https://www.linurs.org/sudoku_solver/index.html?sudoku=...2...57..9738..4....4..9..9....63.4.6.8.5.2..2....4.723.9........753....5..2...
https://www.linurs.org/sudoku_solver/index.html?sudoku=1.........3...59..2...6.173...81.....68...74.....96...624.3...7..76...5.........4
https://www.linurs.org/sudoku_solver/index.html?sudoku=312854..996...3......6.9.3.73....9.....9.2...29638741567..98......7...9.5294318..&pencilmarks=1