I know I talk about Fairy Tail a lot, heck I even run the FT club on here, but that's because it's one of the few animes I've watched and it's one of 2 I've ever finished/caught up on. Overall it's my favorite anime I've watched, so of course I'm gonna have to pick from FT openings, which is really hard for me. Out of the 22 that are out, I'd say openings 1, 3, 4, 9, 13, and 18 are the best. Out of those, I'd have to narrow it down opening 3 being the best, although 4, 13, and 18 are my top 3 favorites.
Edit:
I'm starting to think 18 is better, but maybe that's just my mood. My perception of good music tends to change with my moods.