No idea what the best is, and Lancs is not reknown for slabs due to most quarries giving wall climbing. But... try Trowbarrow? On the Assagai wall there are routes like A Sense of Doubt (E4 6b)and Doubting Thomas (E5 6b). There's also the Asylum wall. They could be what you're after?
Col Kingshott also cleaned up the low extremes at Warton Main and added a harder slab... might be worth getting in touch with him to ask. If you're at Trowbarrow, you'll bump into him because he's found there more frequently than fossils.
Not really the county for slabs, but Cameo (E1 5a) ? Cheat (E3 5b) ? They're both pretty good. Cheat is a bit bold at the top but by then it's easy and really obvious.
As well as the ones mentioned at Trowbarrow there's 'Torture Garden' at Warton Main ( might be bird banned at the moment though) The Torture Garden (E6 6b) which is a bit slabby, good pegs.
There's also Gaskins 8b slab at Stanworth! Spitting Image has a good slab but finishes through a roof.
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