Killer Queen by Queen
To say the least, this song is a nightmare, and arguably the hardest in this game. 34 phrases of hell, most of which have very little room for error. Most notably the "Temporarily out of gas" phrase. That one really sucks.
I had been grinding this song a while back, and thought I could FC it, but wasn't making too much progress on it. Recently, after I picked up Smash Hits and have been playing through the vocals career, picking up FCs and going for the coveted full game FC. I practiced this song more and more, and this came. I've wanted this FC for a long time, and I got optimal, so i'll never have a reason to play this song again.
And yes, I play as a girl character. Got a problem with that? Well then fuck you. :P