I'd go with a quartet of tunes.

Innuendo for its message
BoRhap for its popularity
The Ogre Battle Saga for its breadth and fantasy (I count from Ogre Battle to Funny How Love Is as one tune)
and finally, just to be different, I'd go with Liar. It's their earliest epic, and I think an underrated tune in general. They still played it during their live stuff up until 85/86.