In the first arc, it was just a meet and greet to Fairy Tail's main heroes and their powers. The final boss was nothing to write home about.
The next arc got into Gray's past and had him settle an old feud with a fellow student of his master's, Lyon.
The next arc was against the Phantom Lord arc which introduces Gajeel and Juvia when they were enemies. Makarov shows some his badassery, ending the arc.
Next is the Loke arc where Loke is revealed to be a Celestial Spirit in hiding. Lucy takes center stage as she helps him keep a promise, gaining her two powerful Keys. Kind of short.
Tower of Heaven is a pretty dark arc and focuses on Erza's past and delves into who Jellal is. Erza leaves the guild for a while after the events.
Next, and this is a personal favorite, was the Battle of Fairy Tail where Laxus tries to stage a coup and take over the guild. It goes into why he's such an asshole, but it ends where he's excommunicated. The guild silently send him off in forgiveness.
The Nirvana Arc is where the story gets more focused. Fairy Tail and a couple other guild team up to take down a major Dark Guild. This is where Wendy is introduced and joins Fairy Tail by the end of the arc.
The next is the Edolas arc, another personal favorite, where the team is sent to an alternate universe. Turns out there's a huge conspiracy and comic relief, Happy, was supposed to play a major role in it. the mystery of rarely seen Mystogan is finally revealed here too. Carla and Lily make their appearance here as well as the dramatic return of Mira. This one is an emotional roller coaster.
The Tenrou Island arc is a big one. It reveals who Zeref, one of the two main antagonists of the series, is. It was supposed to be a rank test, but Demons appear to sabotage it as well as other folks for personal reasons. It also reveals that Gildarts and Cana are blood-related and introduces the second main antagonist, Acnologia. Mavis, the ghost loli guildmaster, also makes her appearance here. The arc ends in a timeskip.
The next one is a small one called the X791 Arc which is when the team reappeared from the timeskip seven years later and get caught up on what they missed.
The Grand Magic Games Arc is a fun one. It introduces Sabertooth, a rival guild, and Raven Tail, lead by Laxus's asshole father. This is kind of a long one. It starts off as a magic tournament and introduces new rivals, friends, and villains (Minerva being chief among them), and it goes into a an alternate future arc where one of the characters, Rogue, tries to manipulate the present. Future Lucy was able to warn the team in time and help save the day, but dies at the end joining the rest of future Fairy Tail who also died.
The Tartaros Arc is a doozy. It starts in Flare's village, former enemy turned ally, where they agree to rescue it, learning about E.N.D. and Tartaros dark guild, which is full of Zeref's demons. Minerva, always the thorn in everyone's side, also joins to become a demon. Their goal was to erase magic off the face of the planet. For some characters, they even reconnect with their past, such as Gray encountering his father, Silver, and the team meeting power-hungry Jimma, the original Sabertooth guildmaster and Minerva's messed-up dad. Igneel and the other dragons return to help but the kabosh on that plan near the end of the arc. Igneel even fights Acnologia. He loses, but manages to sever Acnologia's arm. While the dark dragon splits, Igneel and the others reveal what really happened to them and properly die in front of their surrogate children. All in all, this arc is one of hell of a warzone on many MANY fronts. In the end, it's revealed that E.N.D. is Natsu and that Zeref, who wants to die but can't, hopes Natsu will be able to kill him since the other demons he created couldn't. It's also shown by now that Zeref is a Black mage with not much control over his magic, suffering from a kind of split personality. Because of the damage done, physically and mentally, Fairy Tail is disbanded and everyone goes their separate ways, leading to another timeskip.
The next one is the Avatar arc. Personally, this one was overkill. It showed the reunion of the main team members, except for Gray who joined the Avatar guild cult (later revealed to be undercover), abandoning Juvia. Carla, Wendy's cat, has the ability to turn human-ish and the other heroes show off their new powers as they pummel Avatar to oblivion. Despite their goals, they weren't that noteworthy.
The Alvarez Empire is the final arc and easily the longest. Fairy Tail, now reunited and rebuilt, found Zeref's kingdom (where he is beloved), and take the fight to him. His followers, the Spriggan, are stupid strong, of course. Brandish is one worth noting since she has a grudge against Lucy and her mother. Another is Irene, Erza's mother who is one hell of a tragic story. It's also revealed here that Zeref and Mavis were lovers in the past and Mavis's one regret was not being able to "save" Zeref. We also get to see Acnologia human form, though he still lusts for war and destruction. Zeref tells Natus that he is E.N.D. and technically Zeref's younger brother. Zeref reveals everything here: Natsu's true origins, the motivation of the dragons, who the demons are, and what his end-game is. EVERYTHING! For Natsu, killing Zeref would also mean Natus dying. During the war Aquarius, Lucy's first Celestial Spirit, sacrifices herself to help turn the tide of a battle. On the other side of the battle field, a one-armed Acnologia is also killing everything. At some point, Natsu loses himself and becomes the demon E.N.D., but Erza manages to snap him out of it. More fighting and more resistance later, Zeref and Natsu fight at Fairy Tail guild and Acnologia arrives at the continent ready to slaughter again as the other guild members resist him. Mavis arrives during the battle between Zeref and Natsu, where Zeref becomes borderline a god, though still defeated by Natsu (barely). Mavis and Zeref use the last of their magic to suck Acnologia in a time-space wormhole and the lovers both die afterward undoing their magic on each other. Unfortunately, Acnologia is too damn strong and breaks out of the wormhole, absorbing the time magic. The team splits where the Dragon Slayers throughout the series unite and go inside Acnologia to kill his spirit, while the rest of the mages of the world kill his body. The final chapter has it where Levy is implied to be pregnant with his kid (confirmed in this series), Gray enters a relationship with Juvia, Lucy is an award-winning author, (we even see a cute scene of lookalikes of Zeref and Mavis meeting for the first time and flirting). The chapter ends as Lucy thanks Natsu (with natsu killing the romantic mood) and they both leave to go join the others on the 100 Years Quest.
