Do not forget the Cwm Eigiau Horseshoe, that i recon comes very close to the Snowdon Horseshoe.
Then there's the Cwm Llafar Horseshoe, and one ive made up myself, a traverse of Diffwys via Bwlch Yr Rhiwgyr, and on towards Yr Lethr and finishing off on Moelfre's summit.
That' ones quite a challenge, not very far distance wise, but far enough, and after a bit of rain, fishing waders would come in handy below Yr Lethr, one of the boggiest locations next to the Carneddau's Caseg Valley i know of.
Snowdonia has loads of classic walks, but my no one favourite is the Eigiau classic, and if the pea soup is down, you can spend an eternity on the summit of Llewellyn.