You can take individual arcs, and find other works that surpass it. But understand that when complete, One Piece will be at least 15 years in the making, span dozens of arcs and cover hundreds of characters. The level of consistency and sheer scale of the One Piece world to this point is pretty much unmatched in anything I've read. I've yet to read the four books in the Tolkien series that shit on my shelf, but I can't imagine as loveable or enjoyable a cast and environment has been made in that series. At least, not if the movies are anything to go on.
Oda's got a gift for storytelling. The manga is merely at its halfway point and it just keeps getting better. That's 10 years of awesome. PEACE.