I'm surprised at people saying Marineford and Imple down...both lacked the entire Straw Hat crew.
For my money, it's
1) Water 7 - Enies Lobby - and for me this goes all the way until they name the Thousand Sunny and flee Water 7.
2) Alabasta - and I'll cheat here and say this is everything from Reversing Mountain to Alabasta, maybe excluding Drum Island because that one really doesn't link to the entire Baroque Works/Alabasta plot. But Tiny Garden and all that other build up is part of it for me.
3) Skypia - again, I'll include the preliminary stuff here on the island with Cricket. This was pure "adventure" for the sake of it.
4) Not really one arc, but I consider everything up to Reverse Mountain to be one big arc, call it "East Blue Arc". Yes, I'm cheating huge here, lol
5) Shabondy-Marineford. I consider this all one big arc again. Shabondy is so short that it's not really a whole arc. This one loses out for me because it lacks the whole crew, but I could see putting it up as high as 3 for me...just hard to justify any higher.
6) Thriller Bark. A slight arc with some great moments. Not bad by any stretch, and I like it's brevity.
I find it hard to rank post-time skip, the pacing has been off the entire time. Right now this Wano arc is feeling more like pre-timeskip though, so this one might go up on my list above. I've liked other arcs reading in one shot, but as a week to week thing, they drag so much (also, please no more "the fight lasted 22 hours". That just feels like nonsense.)