Best: Tenrou Island arc Every chapter and episode was brilliant and entertaining. The dialog was funny, and the fights were above average. This ark also advanced the story farther than just about any ark. It showed how strong Fairy Tail was by fleshing out Gildarts and Makarov, while also introducing future enemies, Acnologia and Zeref. In addition, it also helped explain Fairy Tails past by introducing Mavis and Hades.
Worst: Daphne arc Filler crap is crap
Bipolar: Grand Magic Games arc This ark had some amazing fights, but the setting was awful. On top of that, it kept flashing back and forth between the GMG and The Rescue Team. The fights between The Rescue Team and the Garou Knights was just lame in comparison to the GMG, making it incredibly difficult to watch.