Ooh interesting thread!
Setting - Close tie between Water 7 (i loved the water town!) and Mocktown (the only true piratey town to date. lol)
Flashbacks - Drum Island; I loved Chopper's flashback! the endscene with the fireworks at the end was just remarkable.
Action - Marineford for me. Too much Whitebeard, Akainu and Aokiji goodness in one major battlefield. And then we still add everyone else in. This arc rocked.
Adventure - Arabasta, from the time Vivi meets the crew as Ms. Wednesday all the way to the final battle. Honestly, no other arc featured them travelling together as well as this arc did.
Characters - Water 7 + Enies Lobby. The characters, some of their eventual reveals, the VAs, Luffy's growth and the best thing in this series: Going Merry!
Plot - a close tie between Marineford (Ace's eventual death) and Enies Lobby (Strawhats standing perched on the roof, burning the WG flag + Robin's cry for help + Going Merry to the rescue)
Humor - They are all really good. No comment.
Villains - AKAINU. Pure badass. I also like Blackbeard as a villain - he is pure evil, no honour, no decency, just pure ambition and strength. Anyone else noticed he took 2 shots from whitebeard at point blank but could still stand and fight Sengoku?
Mystery - One piece doesn't have much of this does it. I guess the whole Robin saga during CP9 arc.
Drama - Where do i begin. I love all their backstories and Ace's death, but Going Merry's rescue and death just takes the cake for me. Chopper's backstory is a close second.