The Final saga has been peak and it's obvious how much planning went into it. I would be fairly surprised if he actually botches it from here on.I forgot about Egghead. I haven’t read that far yet but from what I can tell it’s probably around Zou Arc tier too. I don’t think Oda can make any good overly long Story Arcs anymore though.
It's an "arc" like Davy Back Fight and that Jaya stuff before Skypiea is an "arc," but I think the consensus is that it's basically part of Fishman Island.
I think the most impactful sub-arc or sub-saga was Impel Down though, even though it just leads up to the Summit War arc. I really liked it because Magellan wasn't all that powerful yet felt like an unstoppable force and you got to see side villains more fleshed out in the midst of all the danger. Part II arcs are shit so I stopped keeping up after Dressrosa though.