Man, this is so tough. Nova Era is definitely up there. I remember when I first heard of Angra some 7 years ago, I went to their site and that song was playing. I think my exact thought was "Holy FUCK! What is that song!?" So that one's way up there for me. Along those same lines, Almah's Fragile Equality is pretty amazing. It's the song that sold me on those two albums.
As for underrated power metal bands, I have to toss in Twilightning. I love their stuff, except for Swinelords...that's almost not even the same band.
In the Fervor's Frontier is pretty much awesome:
Iced Earth's Dracula has some of Barlow's craziest high notes. So that's up there along with Burnt Offerings for me.
For plain badassery I put in Sabaton's 40-1 and Ghost Division.
I love that someone put in Pantera's Power Metal, I was going to do the same thing myself, for shiggles.
I've clearly overshot my limit of 1. If I had to pick just one of those, it would be the Twilightning. It's just so very...power metal.