3-8 (one with the big fish)
5-6 (one with the turtles)
3rd fortress in world 6
7-4 (water level)
7-8 (one with all the plants you have to dodge)
2nd fortress of world 7 (almost impossible)
8-1
8-2 (hardest jump in the game)
fast moving battleship level in world 8
other (include your choice, imho)