You won't hear too many arguments if you call Mount Washington the hardest climb in the world. It probably isn't, but it's so hard that no one's really going to think it's an outrageous claim or anything like that. 7.5 miles, an average gradient of 12%, portions unpaved, potentially freezing temperatures at the top, and horrible winds make this it one of the true cycling monuments. Unfortunately, it can only be climbed twice a year at the Mount Washington Auto Road Bike Race where they extort $350 for the privilege of climbing it, or the bargain-priced Newton's Revenge, which only costs $300. Most racers, even top-notch, find that they need very low gears, usually 1:1, for this climb.
Mount Washington is located in New Hampshire. It is 12135 meters long with an average gradient of 11.8% and a maximum gradient of 16.9%. Find similar hills.