This is a tough one. Davy Back Arc was up until recently my least favorite arc, without a doubt. It was irritating and felt like it served the story very little. In short, it felt like a filler arc. But still, it made me laugh a couple of times, I was still somewhat engaged and cared about what happened, and as a filler arc it'd be okay. And since I watched the anime up to the CP9 arc before I started reading the manga, I thought it was filler the first time I watched it. Thus, I thought it was okay. Yeah, it barely moves the story forward at all until the arc is basically over (Aokiji shows up), which is annoying since the arc before that, Skypiea, didn't do that very much either. But at least Skypiea was good, while DB was meh.
Then there's Fishman Island... Dear shit, was I disappointed in this arc. After all that build-up, all those years, the first thing we see post time-skip, it'll be magnificent, it'll be epic, it'll be all that good shit!!!
Nope.
It introduces some of the most forgettable characters ever, probably the worst villain so far in One Piece, and a really dull plot... This arc had me so uninterested, a couple of weeks could pass before I remembered I hadn't read the manga in a while. That's never happened before. Halfway through, I just didn't care how Oda ended the arc. I just wanted it to be over, move on to the next arc an amaze me again damnit!!!
And he did, even before leaving the island. Because Big Mom and her subordinates were instantly interesting, the way One Piece characters should be. Luffy challenging her was way more exciting than the entire FI arc before that combined.
So in summary:
Fishman Island fails miserably in living up to the hype that it's gotten since the friggin' East Blue Saga as well as following the timeskip, but it does move the story forward. In the dullest, most dragging way possible. Hodi Jones is a terrible villain with no motivation for being evil. The plot of the arc is boring and unengaging. Seriously, I don't give a crap what happens, just let the damn Straw Hats carry on with their journey. The ending with Big Mom and all saves it a little, but I can barely call that part of the arcs plot.. In my head, it's not even part of the same arc.
Davy Back arc fails miserably in following up Skypiea, it doesn't move the story forward, it makes me frustrated and irritated but at least it makes me feel something. You're supposed to be annoyed with Foxy and the fact that Luffy doesn't have an easy time beating him up. It succeeds in that, and it also has a couple of really funny moments. And in the end, I was way more excited in the fight between Luffy and Foxy than in the fight between the Straw Hats and Fishman Pirates.
Hell, now that I've written all this, it's not a tough one at all. I support the nomination for Fishman Island arc, without a doubt.