My absolute yêu thích one though, has gotta be opening six, "Fiesta". It's so....lively! Though I gotta admit, the weird rap at the end of this one is partially why it's a very close second. XD
True, I can't really pick a bad one(out of the ones I've heard) though there may have been a few that were too cheerful/serious when the arc itself was the total opposite, bu otherwise, I loved all of them! Another one that stood out to me was "Strikeback", opening sixteen I believe, though I'm not sure...
That and "I Will Not Bow" bởi Breaking Benjamin were my hàng đầu, đầu trang two songs outside of anime for a long while....in fact, if I think about it. They still kinda are.