"Eternity" is the boss battle theme that is synonymous with Blue Dragon. It plays so often throughout the J-RPG that it's all but imprinted upon your psyche by the time the game is over.
This song was sung by Ian Gillan from Deep Purple. Just let that sink in for a second: the guy who sang Smoke on the Water is singing this. It's not his best performance by far, but the song manages to maintain such energy, not to mention being super catchy to boot, that it doesn't really matter.
"Eternity" will pop into your head randomly for years to come, which is only fitting given its name. It was always a pleasure when this song started playing as I entered a boss battle, and like at least one other song on this list, it never fails to get you pumped up and raring to go!
I might add "Eternity" to my workout playlist now that I think about it...