Hey Synthfreak - I would say that Voyage 34 is one of my favorite of their recordings. It is an interesting concept album and I like the pace and creativity of the first two parts (I and II) especially.
A few other of my favorites would include: Up The Downstair, Signify, Sky Moves Sideways, Deadwing, Stars Die, and Lightbulb Sun. But I seem to like 85-90% of their work. In fact, they have so much out there that I've only heard maybe 40-60% percent of their material - yay!
Anyone else really like PT's early release, Yellow Hedgerow Dreamscape?