WHAAAATT-- I prefer your earlier list pope, and Arlong is not a dumb bad guy. He's not as epic as Croc or Luci, but then, THEY CAN'T ALL BE. Arlong did a lot for building the personality of the series, and I actually really like him as a character for being multi-dimensional, and while having a goofy side, being menacing in his own right.
Eh, I might put Fishman island over Arlong Park just because, while the villains were pretty crappy, the revival of the Strawhat Pirates was epic.
Enies Lobby
Alabasta
Impel Down
Sabaody Archipelago
Arlong Park
Here's a man who read the series. Marineforde belongs nowhere near this fucking list (shame on EVERYONE who put it in there)-- for being the most fucked up arc in plot, in power, in making no sense (in that zero characters who should have had Haki used it), and for being a chaotic mess.
Impel Down I have my reservations just because it connects to Marineforde but... Impel Down is just too great to be held down.
Impel Down introduces Jinbe and Invakov, two of the most awesome epic characters-- and Mr. 2 Bon Clay and Luffy are soooo fucking amazing that there is no way Impel Down can miss a best-of-all-time-arcs list. There is just too many laughs, tears, and epic in this arc-- until Marine Forde shits on it.
And really, Marine Forde would have been great as long as:
-Ace and Luffy's back story was told before and not after, and Ace got a bigger role in the main story before it.
-WB did something relevant
-Characters who should have had Haki used Haki (ie. someone needs to explain Haki fully before Marine Forde)
-Except that the Kuja arc kinda-sorta-already-did that, therefore, there's basically no excuse for Haki not being incorporated into Marine Forde thoroughly.
-If every other inconsistency and BS that happened in the course of the Marine Forde battle was ironed-out before it.
-ie. Marine Forde needed to happen, but it happened WAAAYY to
pre-maturely, making this arc as disappointing as 12-second-sex.
Impel Down however, is consistent and has none of these problems. It's not the fault of Impel down that Marine Forde had all these flaws, so I'm trying to judge it without being influenced by the crap that is the connecting arc.
But yeah, I cannot agree more with popemobile's list here-- every arc is awesome, and in the right place too. Good job.
edit:
Also, there is absolutely no way anyway anyone can rank any of these over Ernies Lobby/Water Seven-- this is benchmark to which all other series should be compared. Lucci and his gang as villains, Robin and Franky's back stories, the death of Merry Go, the birth of Sunny Go-- it goes on and on, and no other arc comes close yet. But then, it seems we all agree on this so lol