It depends what you class as 'must ride' roads....
The Stokesley to Helmsley 'TT' is one that many harp on about but I tend to avoid as there are generally too many knobheads on there on a nice day. It's much nicer to take it to chop gate, take a right & head back over to Carlton (maybe a brew at Lordstones cafe & take in the view) zip down the A172 Northallerton road to Osmotherly (nip off early past Cod Beck res, then over to Hawnby, Old Byland & Rievaulx.
To be fair, once off the main A170/A171 drags, you won't go far wrong in the NYM and don't be afraid to dip into the Howardian Hills - Ampleforth, Coxwold & around Castle Howard are all lovely.
As said, there are some good routes by Dalby forest/Forge Valley/Hackness/Silpho.
Other faves of mine in no particular order are;
Egton Bridge/ Stape/pickering or take a left near Stape on the unsealed road to Levisham.
Castleton/Ainthorpe/Fryup/Rosedale, head up Rosedale Chimney bank to Hutton le Hole or follow the valley to Cropton, Lastingham.
Enjoy. Keep off the main drags, don't spend agrs ploting a route on the 'nav & just meander.
Edit, Interesting comment from sven that the Blakey road is by far the best over the NYM & it goes to show that we're all different and want different things from 'good' roads
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