haha I'm going to make a fool of myself by saying this but... My favorite arc was...
*drums*
The Navarone Arc, yes it was filler. But it was non stop humor and fighting. Like how the crew bonded with some of the marines. Vice-admiral Jonathan was a cool charachter and his wife too. He almost captured the strawhats if it wasn't for a perverted trick from luffy to escape
From the non filler arcs, it's hard to choose between Arabasta and Thriller Bark. Liked the story best in Arabasta. But Liked all the new nakama in Thriller Bark fighting Moria and Oz.