I'm a huge Maiden fan and have seen them a number of times live. I just saw them this Wednesday, and the show was not very good. Bruce was having mic issues and his voice was strained for a lot of the set. I wonder if it has anything to do with how late it is into the tour. The guitar work was off a bit too. Totally surprising. Niko and Harris were aces though.
I like Ghost and have seen them a few times, but they didn't have the best set either. It was an outdoor venue, and the sun was still up when they came on. They seem to thrive in a small/club venue. They're also not a high-energy opening act like I would have imagined for IM. Waaaaaaaay better than Shinedown though, who apparently opened for them in Europe. Puke.
As for my favorite album, I would have to say Power Slave for sure. Solid top to bottom.