Definitely the Cliffs of Dover.
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Just look at all that chalk, waiting to be used as flux for steel manufacturing...
I've been thinking about asking for a while now (and always forgetting to scout around my generated worlds in via the embark screen or Adventure mode, when I'm at home and free to do so...) but does Worldgen generate sea(/lake)-side cliffs like that? I'm rather partial to deep river gorges and steep hillsides and the like, but have never seen anything approaching a "0000*0100"-type situation on the steepness indicator of the embark chooser, where there's water (or even marsh/flat plain, actually), cliff, then relatively flat plateau.
Worldgen is good at drilling rivers down through strata (including obvious course changes leaving dry canyons), for Grand Canyon style situations but I've never seen an obvious sign of the kind of lateral cliff-forming erosion that you need for the likes of the WCoD...
Actually, thinking about it, I've seen plenty of low examples of this where two rivers meet, leaving a concave triangulr lake at the conjunction, usually with waterfalls on each of the two upstream legs, but still nothing that shouts "shoreline", and for more than half a dozen Z-levels.
But I've definitely carved out slopes into cliffs, and I'm fairly sure that with enough effort I could have drawn in a lake/sea-shore towards one, by channelling the shoreline into it, as long as it wasn't so flat a coastal plane to start with that there wasn't a rise to encliffify.