Not a fan of the direction Shingeki No Kyojin went to, but the first half of the series is really great.
And the problem with One Piece is that they are dragging everything too much with the same formulaic pattern all the time. Almost 24 years later of the first chapter and everything important is yet to be told, Oda better pull it off because that's where it matters the most.
I guess my favorite arc ever is the Frieza Saga.