This is actually a very interesting one!

Starting off, my favorite arc in all of Fairy Tail is easily the Edolas arc.

If I'm being honest, a lot of people tend to overlook this arc. I kind of get why. It was put between two of the more important arcs, the Oracion Seis arc and the Tenrou Island arc.

I love this arc because of a few reasons. One is that they pretty much get Isekai'd. And this was back when Isekai anime wasn't very common, so it was pretty cool.

If you don't know this arc, then you're not very far into the anime. The main cast basically have to travel to Edolas, a parallel world to theirs, however magic is almost completely gone from it. And in order to get magic back, the king of Edolas decides to steal it from another world. I'm doing this, he captures everyone in the guild besides Natsu, Wendy, Happy, Carla, and Lucy. It is then their job to save everyone before they are turned into pure magic.

And the whole concept of an alternate world of Fiore was executed very well. They had alternate versions of characters in which some acted completely different from the ones we know, and some act almost identical.

The best contrast and joke in the whole arc was when Wendy saw her Edolas version and saw she had a bigger chest than her. 😂

I'm kidding Wendy, flat is justice!!

It was also the arc where Wendy really got to show what she could do, in terms of fighting anyways. It's also where we got my personal favorite outfit for Wendy... >_<

Short skirt and thigh high stockings, you already see why it's my favorite... 😅

Another reason is the final fight between Natsu, Wendy, and Gajeel vs the robot dragon was one of my personal favorite fights in the series.

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As for my least favorite, this may be a little obvious. The Key of the Starry Sky arc is my least favorite.

I kind of feel the arc really wasn't needed. The same argument could be made for the Edolas arc, however I just found it far more enjoyable than this one.

It just felt kind of bland and it used villains that were already used, the Oracion Seis, even though they had some "new" members. Don't get me wrong, the Oracion Seis are probably my favorite evil guild. But I find it kind of stupid that they used them again.

The whole story I found boring. The only part I liked was when they introduced Lucy's "sister". However the plot twist about her character was just forced in my opinion.

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Anyways, thank you for the AMAZING question!! I really hope you enjoyed this, and don't judge my taste too much