Here's my (rather long) list of songs I think are epic. I know some of these have been mentioned before, but here goes:
"Ode To Joy" - Beethoven
"The Beginning is the End is the Beginning", "The End is the Beginning is the End" - Smashing Pumpkins
"Back in the Day" - Megadeth
"Frontline" - Pillar
"The Show Must Go On" - Queen
"The Will of One", "Vengeance", "The Stand (Man or Machine)" - The Protomen
Thriller" - Michael Jackson
"Fly from the Inside" - Shinedown
"See Who I Am", "What Have You Done" - Within Temptation
"Through the Fire and Flames" - DragonForce
"What I've Done", "No More Sorrow" - Linkin Park
"Welcome to the Black Parade" - My Chemical Romance
...What? Don't look at me like that!
And if anyone doesn't know what any of these songs are, just ask. I'm more than happy to tell you about them.
EDIT: Oh yeah, I almost forgot- "Defying Gravity" from the musical Wicked. Enough said.