The "Hime Hime" song from Yowamushi Pedal. https://www.youtube.com/watch?v=7NGIw5kjcWQ
And, yes, I actually have sang parts of it during long distance bicycle rides like Onoda. Although the songs I tend to sing more are "Breaking the Law" by Judas Priest, "Supercharger Heaven" by White Zombie, Thundercats opening, and...
(I pity you if you don't like the OG Pokemon theme song.) One of my students' first project in his TV/Media class was to create a music video and he chose this song. And let's just say the class LOVED it! No one else thought to have done a song like that.
A few others I like:
Bakuretsu Hunters opening theme (this is a total guilty pleasure anime and manga series for me): https://www.youtube.com/watch?v=kT0xVZtbHuc
Slayers opening theme. So catchy... https://www.youtube.com/watch?v=RQmW-Rylr5c
"Sayonara Stranger Days" the end credits theme from this crummy OVA called Capricorn. Easily the best thing about that: https://www.youtube.com/watch?v=S9J4skId-hE
"Cyborg Mermaid" the end credits theme from the Battle Angel OVA. That song is so haunting. That OVA is why I read, and love, the manga. And, yes, I think James Cameron making a live action movie adaptation is going to ruin it. https://www.youtube.com/watch?v=mOdzvwKxcmQ