Of the actual two choices, Regret Nothing. Autotunage aside, I like that Shu actually put some effort/energy into singing, while Inoue, I dunno, felt lifeless in Ride the Wind, leaving thos awesome background chanters to supply most of the energy.
But since there isn't an actual Poll, and the OP said "best song sung by the lead rider", I'ma going to the truth now.
Every single Tetra-Fang song.