What's the hardest climb in the world?
There are four sport routes with consensus grades of 5.15c (or 9b+), the highest difficulty for free climbing in the Yosemite Decimal System:
Change, in the Hanshelleren cave of Flatanager, Norway. This route was prepared, bolted, and sent by Adam Ondra in 2012.
La Dura Dura in Oliana, Spain was bolted by Chris Sharma and first sent by Adam Ondra in February 2013. Sharma followed with his redpoint a little more than a month later.
Vasil Vasil in Sloup, Czech Republic was redpointed by Adam Ondra in late 2013.
El Bon Combat in La Cova de l'Ocell, Spain was redpointed by Chris Sharma in March 2015.
The hardest big-wall free climb is The Dawn Wall 5.14c, Grade VI in Yosemite, California, first climbed by Tommy Caldwell and Kevin Jorgeson in January 2015 and subsequently climbed by Adam Ondra in November 2016.
Any claim for the hardest aid route is very contentious but Nightmare on California Street A5 in Yosemite, put up by Warren Hollinger and Grant Gardner in 1998 may take the prize. It hasn't seen a repeat ascent.