It'll have to be Fairy Tail's first opening for me, Snow Fairy by FUNKIST as it shows the core cast(albeit spoiling Erza a bit earlier than I would have liked, I needed more convincing that Erza was a boss as I saw Natsu on her level in the introduction) and that flute.
The third opening, Ft. by FUNKIST is also good as the combination of energy and the flute feels like it could fit into the core soundtrack again. Those are the only openings which I could really identify with the series while the endings are pretty meh as far as I remember, which could be the fault of using the same animation type for the ending each time.
It's a shame that they can't help spoiling the next two arcs with half of their sequences anyway. >_>