For me I think it’d be hard to argue against this being their best. Big fan of the one-two punch of Aces High and 2 Minutes to Midnight off Powerslave as well.
In terms of live albums, Live After Death is the classic, but Rock In Rio was my introduction to them and I think stands up well. Brilliant versions of two Blaze-era tracks as well.
Number of the Beast probably their best record but difficult to pick one. A fair few duds over the years, but everything up to Seventh Son is brilliant, and most since Brave New World have been solid. A handful of good tracks in between.