I voted for The Clansman, not because it's the most underrated, but it's the best song on the list.
But having recently dived back into Maiden, I discovered that I actually really love No Prayer for the Dying after having for so long thought it was a stinker.
So I'm gonna throw some other underrated gems into this thread, or at least ones I never hear being discussed:
- Tailgunner
- Hooks In You
- Only The Good Die Young
- Sun And Steel
- Heaven Can Wait
Hooks In You in particular is kicking me in the nuts something fierce right now.