For Fairy Tail to be better than One Piece, each of its anime episodes needs to be around three times as good as its One Piece opposition, as One Piece is three times as long as Fairy Tail currently. (It's already better than Naruto, which is twice as long as Fairy Tail currently, which means every Fairy Tail episode is more than twice as good as its Naruto equivalent.)
Let's look at these things in chunks then -- From the start of Fairy Tail until the end of the battle of fairy tail, the episode titled Fantasia, is where we get to know all of the main characters minus Wendy and their back stories. The similar block of material in One Piece extends until episode 53 where the main crew is gathered in East Blue, we see their back stories, and then they go off into the grand line at the end.
Of these two expositions, which is superior? The answer is Fairy Tail. Fairy Tail has a hell of a lot more happen than One Piece, with a hell of a lot better art than One Piece could ever hope to have. One Piece certainly has some great episodes in this arc, like Nami's flashbacks, Zoro's flashbacks, Sanji's flashbacks, and the good fights on Usopp's island and at Arlong Park.
But Fairy Tail can match that punch for punch. We get Erza's flashbacks, Gray's flashbacks, Natsu's flashback remembering Lisanna, team Laxus using clever tricks to keep even with the entire rest of Fairy Tail combined, Phantom's all out war with Fairy Tail where we get to see everyone in the guild show their stuff, Lucy's fated meeting with Natsu at the very beginning and their subsequent adventures together where they become steadfast companions, Natsu rescuing Lucy from Phantom Lord solo when all hope looked like it was lost, Elfman's flashback of killing Lisanna, Lucy saving Loke from the spirit king's judgment, Natsu pretending to be a bullied cat to defeat the all-powerful enemy Milliana, the vision of Erza's funeral when she merges with the ether-nano, Juvia wanting to be accepted as part of Fairy Tail to Cana, Levi crying out to Laxus that his grandfather was at death's door . . . three times as good? I say no problem. Even with Nami asking Luffy for help. Even with that scene in mind, yes, Fairy Tail's beginning was three times as good as One Piece's beginning.
One Piece's next arc is Alabasta. It goes all the way to episode 135. Within that time period, they meet Vivi, Zoro fights 100 bounty hunters at once and wins, Zoro becomes a gallant wax figurine but is saved despite the amazing combo of Mr. 3 and Miss Goldenweek which almost had Luffy licked, Nami gets sick and is given tender care all the way up the snowy mountain of Drum. Chopper's incredibly sad back story is revealed, as is his incredibly sentimental departure from Doctorine. Luffy meets Ace for the first time in many years. Vivi and Luffy fight about what their goal should be. Luffy beats Crocodile, and the others also do well in fights beyond what you would expect from each of them. Vivi says goodbye to the crew in a tearful farewell. These are great moments, no doubt. Is it posible for Fairy Tail to have three times as many great moments in the same period of episodes?
In the same period, there's budding romance between Lucy and Natsu, we meet Wendy and Charle for the first time, Natsu tries to take out a sleeping Midnight in a total cheap shot, Natsu takes on Cobra and Master Zero in really roaring fights, Wendy says goodbye to her homeland, Gildarts is introduced, the guild is turned into magicite, Natsu and Wendy adventure in a parallel world without magic alone together, unlimited smiles competed with unlimited magic as the proper path to the future, Happy and Charle learn about their origins and form a bond of friendship together, Wendy, Natsu and Gajeel team up to take down a dragon mecha, Erza fights a wielder of the 10 commandments from Rave, Lisanna is miraculously returned to Fairy Tail, Natsu fights Gildarts, Urtear reveals her true power, Meldy fights Juvia, everyone works together against Purehito, then everyone works together against the even mightier Acknologia. In addition, Mavis is introduced and a momentous time skip occurs.
That's about twice as many good moments as One Piece, but not three times as many. In that case Fairy Tail falls short of the goal this time.
How about the next arc? In One Piece we have now reached Sky Island. During this arc Luffy faces down Bellamy, beats Eneru, gets into a really annoying encounter with police, Wiper proves his mettle, Nami trusts Luffy to protect her, Montblack Norland's flashback is recounted, Chopper is told that he must get stronger if he is to make demands of his opponents, and Nico Robin shows off her skills. This goes until episode 206.
How about Fairy Tail in that same period of time? This covers the entire Grand Magic Games of Fairy Tail, in which many a good thing happened. The badass rival guild Sabertooth is introduced. Lucy fights Flare. Wendy cries because she can't compete. Wendy is saved from kidnapping by Natsu. Kagura fights Yukino. Erza takes on all 100 magic beasts at once. Wendy fights Chelia. Natsu beats Sting and Rogue together. Natsu lectures Sabertooth's guild master over his treatment of Yukino with biting logic. Wendy, Natsu, Mirajane and Lucy fight the Garou knights. 7 dragon slayers face off against 7 dragons. Natsu beats future-Rogue. Urtear turns back time. Future-Lucy dies and goes to heaven. Urtear becomes old. Yukino and Lucy work together instead of trying to take each other's keys. All the guilds attend a hilarious theme park. Wendy and Chelia become friends and hang out. Fairy Tail returns victorious to Magnolia.
Again, Fairy Tail is twice as good as One Piece during this time, but not three times as good. At this rate Fairy Tail will just keep falling behind.
The only thing left in Fairy Tail's anime is the Tartaros arc, which still isn't done yet. Even if all the remaining Fairy Tail manga were animated, it probably wouldn't exceed in length the Water 7 arc of One Piece. So further comparisons are currently impossible.
There seems to be a lot more One Piece manga planned than remaining Fairy Tail manga planned, so that 3:1 ratio in length isn't likely to change. Unless the later half of One Piece is drastically worse than the earlier half, it just feels impossible for Fairy Tail to ever catch up.
In addition, there's no telling when the Fairy Tail anime might be cancelled, whereas the One Piece anime remains evergreen popular and will definitely not end until the end of the manga is reached. This gives the anime a huge advantage even if the Fairy Tail manga could somehow overtake the One Piece manga.
In both manga length and anime length, Fairy Tail has surpassed virtually all of its competition, but compared to One Piece it still just isn't nearly long enough to compete on even ground. I feel like Fairy Tail is ending soon, so it has basically no chance to catch up with One Piece in the future. The only thing Fairy Tail can hope for is that all future One Piece will suck, which would render the length advantage moot. But since that would be a tragedy for the world I certainly don't want a victory such as that to play out.
I'd love for Fairy Tail to beat One Piece and become #1, but the wall is just too high. One Piece is just too good. Combined with its infinite length, it feels almost impossible for anyone to compete with no matter how far into the future we project. If even Fairy Tail, which has over 200 episodes to work with, can't topple One Piece, who could? If 200 episodes aren't enough, who could possibly marshal an even greater force and fight evenly with One Piece at this point? Even if a new story started now and eventually became longer than One Piece after 30 years of serialization, even if said fabled show actually is better than One Piece in the end, no one alive today will be around to witness that moment. I think we have to bow ourselves to the inevitable. One Piece is immortal and invincible. It is the work of art that will dominate the entire 21st century. Likewise, I don't see Fairy Tail or Naruto losing to anyone in their positions of 2nd or 3rd place either. They have an enormous lead in length over all possible competitors, while having made good use of their length unlike Pretty Cure or Dragon Ball. That killer combo is going to keep all competition away indefinitely. Unless, maybe, Sword Art Online is adapted into an anime in full all the way to the end of the books, whenever that will occur. Then SAO would have both the length and the quality necessary to compete even with One Piece. I can't imagine anyone else even coming close.