Epic for me will forever be synonymous with The Mars Volta's "Cassandra Gemini". However, others I like are:

Norma Jean - Pretty Soon, I Don't Know What, But Something Is Going To Happen
Explosions In The Sky - all of "The Earth Is Not A Cold Dead Place"
Protest The Hero - all of "Kezia"
Queen - Bohemian Rhapsody
Radiohead - a back-to-back playing of "Kid A" and "Amnesiac" that I indulge in quite often </nerd>