Piece of Mind has some excellent highs, but there is also some filler here and there, like Quest of Fire or Sun and Steel, though its highs are amazing (Where Eagles Dare, Revelations, Flight of Icarus, To Tame a Land).
My favourite is Seventh Son of a Seventh Son, though the sound is a little different, because they've started incorporating synths. Somewhere In Time has a similar sound is is fairly highly regarded as well, though, again, some filler keeps it down.
If you want to check out some newer stuff, which is very good consistently, but a little samey over the course of one album, try A Matter of Life and Death. It contains my favourite IM song, The Legacy.
I'd go the following order:
Piece of Mind/The Number of the Beast (core IM sound)
A Matter of Life and Death (similar, but a little different, and awesome all the same)
Seventh Son/Somewhere in Time
Hope that helped!