#5: Thriller Bark
#4: East Blue Arc (Saga?)
#3: Jaya Arc
#2: Shabaody Archipelago: coolest, most colorful characters in a very long time. Plus, we got to see another Admiral!
#1: Marineford. The anticipation for this arc was, by itself, enough to get me back into One Piece after a six year hiatus. Plus, it introduced some of the coolest new characters, had some of the craziest plot twists, and had some of the best "WOW" moments in all of manga.