I don't know what is the "most epic song ever," but I do know several epic songs.
I will stick to actual songs. Movie soundtracks and classical music (including opera), are inherently epic and we would be here all day if I actually included those in my list.
I will start with "2112," a hard rock/metal/prog masterpiece from Rush. Listen to the whole song and you might get why this 20-minute epic is so highly praised by Rush, Prog, and Metal fans the world over.
Part 2 and
Part 3
"...And the meek shall inherit the earth."
This song takes place in a science-fiction-inspired future where liberty is absent and the "Priests of the Temples of Syrinx" oversee all human activity. A member of this society discovers a guitar and makes music, but the priests decry his discovery. Dejected, he returns home and has a dream of a grand future where the creators of the instrument return to their home planet and liberate all of the people dwelling there. Unable to bear his reality, he kills himself, but soon after the Elder Race actually
does return home and tear down the temples. It's a great individualist treatise, inspired in large part by Ayn Rand, and the music flows very well. The climax is unbelievable.
"I stand atop a spiral stair
An oracle confronts me there
He leads me on light years away
Through astral nights, galactic days
I see the works of gifted hands
That grace this strange and wondrous land
I see the hand of man arise
With hungry mind and open eyes"
The amount of passion and emotion in this song, as well as the subject matter, makes it nearly unparalleled (in my view) among metal/rock epics. The riffs are incredible, and the lyrics, while cheesy, convey a powerful story. Geddy Lee's singing is the subject of much scorn for people who dislike Rush, but I think his shrill vocals actually benefit this song, and all of the instrumentation is perfect. This is an unabashed hard rock, damn near heavy metal rock opera, so shameless that it actually
starts with a thunderclap. I love it.
There are other examples of "epic songs," but I'll let my opinion rest there for now.