I think I get what wcf is getting at - a show that grips you from start to finish with no weak points or lesser moments. The kind where 10 minutes passes in a flash. Yeah, certainly 'perfect' is in the eye of the beholder. To me, it means a 'perfect' balance of emotion, sophistication, and accessibility, and a 'perfectly' unified yet contrasting program that builds and builds in musical and dramatic tension... higher ... faster ... louder... until... until.... yes.... yesssssss... : that perfect moment of musical and dramatic ..ummm.. release.
Certainly '84 Cadets is the top of the list.
Others (mostly the usual suspects):
BAC '00
Several Phantom years including '89, '96, '06, others that come close.
SCV '85,'99,'04
Spirit '80
Suncoast '85