My nightmare hill has always been Rawson's Raike, in Ramsbottom. On an MTB you can climb the tarmac road then get a short break before continuing on a steep narrow lane right to the Peel Tower. I've watched a hill-climb there and seen roadies arrive at the top and barf up.
The lane from the main road (Cross Lane) is passable on a road bike, and if you carry on on Holcombe Old Road, rather than turning uphill for peel tower, it's still passable on skinny tyres, but cobbled to enable Paris Roubaix Pave type indulgence!
I personally find Mytholm Steeps in Hebden Bridge (aka Church Lane, Rawtenstall Bank) very similar but that little bit harder.
Also in Hebden Bridge is The Buttress 30% with rough cobbles and moss, which I've now decided I can't get up with standard gearing and a 25 cassette.
Lots of challenging double chevron climbs round here, couple of others in Hebden Bridge, a nice set going south from Sowerby Bridge, includung a 20% railway bridge . Then there's the two climbs north from Meltham, and the 5 or so north from Slaithwaite. There's a couple of killers in Luddenden too, one of which is in 'the book'. And then there's plenty of less steep but longer climbs, A687 Todmorden to Bacup is a good example. Greenfield to Holmfirth and the reverse are long drags in an almost alpine sense, and Wessenden Head is I think harder than Holme Moss. I also find the south side of Holme Moss a harder climb than the north.