The road out of Robin Hood's Bay used by the buses towards Scarborough has a steep hill along Sled Gates with a sharp bend halfway up which is an interesting rider on a double decker ! The road sign on google streetview shows 25%.
I came here to say this. That hill is an utter brute - and even more "fun" if the X93s pass at the corner, as happened last time I used it. Arriva got late-model (ex-dealer stock) B9TLs for the route and I can see why. That said, Coastliner's B5TLs seem to cope surprisingly well with the brutal climbs over the Moors on the 840, although I suspect experience is needed getting over the narrow bridge at Goathland Station and then getting the acceleration right around the corner and up the bank out of the village.
Where I am in London nowadays, there are three steep (for London!) hills near me that are regularly used - Muswell Hill has the frequent W7 and 144 (11%), Highgate Hill (143, 210, 271, W5, 10%) and then Highgate West Hill -12.9%, and the 214 goes up and down 24hrs a day. The latter converts to electric BYD/ADL E200 MMC shortly - the current E200 and particularly the older DLDs that were the mainstay of the route for about twelve years, were absolutely done-in by the route.