A couple of the first posters mentioned Infinite Dreams ... you Maidenfans newbies may be interested to know that on this forum, that song is well-liked. Definitely not underrated by the members here; it made the top 10 in the Survivor game.
Lemme pick one from each album...
IM: Strange World. Most of the album is well-regarded, so for underrated it's SW or Charlotte ... and Strange World is better.
Killers: Purgatory is a gem.
NOTB: Invaders, which I've always thought is one of Maiden's stronger openers and would have rocked live, if they had ever done it.
POM: Quest For Fire. Great 12/8 groove.
PS: Duellists, for that amazing dual guitar solo section.
SIT: Sea Of Madness. Such a great song.
SSOASS: I don't think there are any underrated songs here. The less-respected songs are in that state for good reason.
NPFTD: Run Silent Run Deep. I could also go for Mother Russia. The rest of the album is a stinking turd.
FOTD: Chains Of Misery. Never seems to get much love, but I think it has a great chorus at least.
TXF: A couple come to mind: Look For The Truth and Judgement Of Heaven.
VXI: The Angel And The Never-Ending Chorus. I make fun of this song, but I really do love it, even the endless chorus.
BNW: Nomad. I've stated my love for this song, especially the Beckett section, many times.
DOD: New Frontier. The riffs fucking rock. And since it is not humanly possible to rate Paschendale high enough, Pasch is always by definition underrated.
Paschenfuckingdale! Say it with me, brother! Paschenfuckingdale! Can I get an Amen! Paschenfuckingdale, Amen!
AMOLAD: Out Of The Shadows. Always reminded me of Bruce's solo stuff.