I'm just gonna run through these in order and see if it I land on a precise ranking at the end.
Fishman Island
Severely underrated by the community. Beautiful art from a refreshed post-holiday Oda, some big contributions to the series' lore and worldbuilding, and themes of generational trauma and inherited hatred approached with a surprising amount of nuance. Hody's lacklustre crew definitely holds it back though.
Punk Hazard
A foundational arc for the new world, setting up science and alliance plot threads that are still playing out to this day, but it somehow ends up being totally unremarkable when taken on its own terms. Fun moments for returning characters like Law and Smoker, and I enjoyed the final set piece with the gas flooding the lab, but so much of the rest ends up being forgettable.
Dressrosa
This arc is Wano Jr - a bloated epic battle story with high highs, low lows, and a feeling that Oda bit off more than he could chew. The runaround of Operation SOP combined with the then-new break schedule had me questioning the commitment to the weekly read. The moment Usopp took out Sugar at the end of that sequence convinced me I was in it for life. Somehow managed to stick the landing after everything.
Zou
I honestly can't think of a single thing worth complaining about here. It was a fun twist on the formula to have half the crew already arrived to do a standard One Piece arc and then put together the details of that adventure through flashbacks while exploring. The reveal of the Lode Poneglyph leads to probably the most information about the eponymous treasure we've ever gotten in one place and even looking back on it makes me feel excited.
Whole Cake Island
The most successful big arc of the New World in terms of structure and pacing. Oda reigned himself in from the side character bloat of Dressrosa, drawing all of Big Mom's kids but making a clear distinction between the ones that were actual characters and the ones that were just set dressing. Loved the big set pieces of the wedding and naval battle escape, as well as the GoT-reference wedding assassination plot.
Wano
Bigger and more full of characters than anything that had come before, with pacing that reflects it. Kaido's first appearance, Oden's flashback and the start of the rooftop brawl are going to go down as all-time One Piece highlights. The Gear Five reveal is a huge pivot that will define everything still to come. But unlike the last flawed but lovable superarc, it doesn't stick the landing, instead backloading most of its biggest issues to leave a sour aftertaste.
Egghead
Great fun so far, with the retro futurist aesthetic of the setting feeling like the setting Punk Hazard's lab wasn't interesting enough to be. While I'm keeping my expectations in check for action in what will probably be a breather arc, my hopes are high for devil fruit and history lore, and keeping the worldbuilding machine chugging along.
I might rank the completed arcs
1: Whole Cake Island
2: Zou
3: Dressrosa
4: Wano
5: Fishman Island
6: Punk Hazard
Yeah, I have more positive things to say about Fishman Island and more negative ones about Dressrosa and Wano, but I'm a sucker for ambition and epics. A story that aims higher gets points just for trying.