Hardest solos I've heard of were back in the early '90s when Alain Robert and Jean-Christophe Normand both soloed proper, big 8bs. Which I reckon makes them the first E10, possibly even E11, ascents.
I think the route Robert did barefoot was Cauchemar d'Elephant at Buoux, 8a.
After his 8b solo, Normand decided to train hard and solo the 8c at his local crag. He trained so hard and dieted so extremely that his liver failed and the doctors told him there was nothing they could do and he would be dead within 6 months. He wrote a very moving letter to Vertical saying goodbye to the climbing community. At the time he could barely get out of bed. He went on to make a complete recovery and has since climbed Font 8b.