I voted for Phantom Of The Opera, because it is a eternal classic and one of the all time greatest heavy metal songs.
Victim Of Changes is great, but never really been a song that I like so much. Priest are not a band that creates so many epics.
On the other hand, Princess Of The Dawn is superb (on close second right after POTO) - the whole melodic riff, the solo, everything is fantastic. An all time classic.
Crusader is good song too. Again, Saxon do not creates so many epics, but this one is for sure very good. Amazing intro, chorus and brutal riff.
I also considered to add more early epics (from other bands) like Battle Hymn, Stairway To Heaven, War Pigs, Child In Time etc, but my first idea was to be 4 bands, so I sticked to that.