Oh, well if that's a criteria than there's only one answer for me:

https://www.youtube.com/watch?v=XU4JWEYuRVM

Also:

https://www.youtube.com/watch?v=Nbq4yBgNVgk

OOO Combo Change Medley. It took all the form-specific themes and puts them together in a fantastic medley that gets more awesome with each combo.

https://www.youtube.com/watch?v=JJmPG5Lv_Zc

^Yeah! Combo Change Melody rocks.

Also also:

https://www.youtube.com/watch?v=1AXdAC8z9HE

ALSO ALSO AAALLLLLAAAALLLALALALALASSOSOOSOSOOOOOOO

https://www.youtube.com/watch?v=KoaAjv7F4hE

One that I really loved at the current moment was Agito's Theme:

https://www.youtube.com/watch?v=yV8ntRpm_CI

I loved it during the opening and during either the final episode or movie it played during one of the fight scenes instead of the other two songs and it was just so badass. Just made the fight scene so much better between everyone.

It's very hard for me to pick just one but since I normally put too many on other lists I'll pick just one here even though I love tons and use them for my alarms lol. Also this post doesn't need 15 video links from one person lol.

Oh, I almost forgot this one. How can leave out the song that inspired the creation of one of my best characters. :)

https://www.youtube.com/watch?v=DF6JbNG9zSo