Personally, I think that One Piece is a better anime than Fairy Tail because it has a better plot, the fights are more dramatic (and less predictable) and the villains are not repetitive, like most Fairy Tail Villains. Subsequently, the art for Fairy Tail has been ripped off of One Piece which makes it unoriginal. Therefore, I consider One Piece to be a better anime than Fairy Tail.
One Piece is a better anime than Fairy Tail because One Piece has a better plot. In each arc the villains are different and have unique back stories. Also the fights in One Piece are less predictable. Contrary to that, Fairy Tail fights are very predictable. It is just Natsu who "faces off" with this villain who is supposedly "super powerful" and insults Fairy Tail. Then in round 1 Natsu looses to the villain and then he uses "Nakama Power" and receives a cool power-up and defeats the enemy. In One Piece Luffy does not rely on his his "Nakama Power" but instead uses the neurons in his brain. For example, when he fights Crocodile in Alabasta Arc, Luffy did not rely on his friends but instead on water which "neutralizes" the sand. Personally, I do not mind if "Nakama Power" is used a battle but the characters in Fairy Tail use it SO many times that it is annoying and sickening.