1. Phantom arc - back when Lucy was a good character, great build up to the story and when she was an actual MC
2. Tenrou arc - very good villain(s), Gildarts fighting for real, Laxus' epic entrance though this arc is also known for the two most BS fights in the series, Erza vs Azuma and Natsu vs Zancrow
3. Tartaros arc - great villains(especially Mard Geer)but also known for one of the most BS fights in the series, Erza vs Kyouka
4. Tower of Heaven - great story for Erza and Jellal was a good villain
5. Battle of Fairy Tail - Laxus being a badass and his development was good and also, since Fairy Tail cannot be beaten, they beat each other instead
6. Grand Magic Games - third worst arc ever with mostly off paneled fights(Erza vs 100 monsters, Laxus vs Jura), hype destroyed(Sting, Rogue, Minerva, Kagura), trolling Mirajane
7. Avatar - second worst arc ever with one shots everywhere, why did this arc even existed
8. Alvarez arc - worst arc ever, one shots everywhere(Natsu), ass pulls, PoF shit, Spriggans getting shitted on, Mavis being the
genius tactician, Zeref doing absolutely nothing, Mirajane getting trolled, unnecessary fanservice
Honorable mention: Key of the Starry Sky arc is between Battle of Fairy Tail and GMG arc and that other arc about Celestial Spirits going wild is only above Avatar.