Doffy's backstory was fucking amazing, but I think Moriah's history with Kaido is the most interesting of the OP villains (like apparently for a Warlord he easily rivaled Kaido, how is that not badass?). Yeah, everyone's connected in some way or another, but Kaido fucked Moriah up so badly it left him with some awful PTSD. So he plotted his revenge for years (and it's a brilliant plan, too), he was just being a lazy lard-ass until the Straw Hats came along.
I'm just super excited to see where Moriah will go from here. He'll sadly not land the final blow on Kaido (though maybe Oda will surprise us), but I hope he'll at least be there to witness his defeat and find some peace.