Ok, so. My initial thoughts went to this:
I still get goosebumps every so often when I watch it.
But I should give some credit to the Wild ARMS series. Mostly 2nd Ignition and the 3rd:
(Disc 1 opening)
(Disc 2 opening)
(Wild arms 3 1st opening, I personally don't like lyrics much, so the whistling melody stands out more this way)