For me, the nostalgia factor plays into the '86 to '89 Petra time period. I was a teenager, saw them live on the Mall in Washington, DC in '88 and was heavy into them at that time. I was actually disappointed by Beyond Belief when it came out, though I've come to really enjoy that record over time.
The Volz era, while not nostalgic, is still very enjoyable to me. I love many songs from the Never Say Die thru Beat The System time period.
I totally agree with Pouncer - favorite Petra song is quite easily "It Is Finished". The song's atmosphere of urgency, thunderous drumming, and piercing lyrics all add up to one great song. I even love the keyboard outro. That song is the culmination of Petra in my mind. They never topped that track.