I'm not familiar enough with the whole back catalog yet to give a ranking. I really just started heavily getting into Priest probably in 2000, with Halford's solo record. I sort of went through this period where I told myself I was going to familiarize myself with all the influences of Queensryche and dive in. I got Maiden first, became a total hardcore. Then it was Pink Floyd, and while they took me longer, and I wouldn't consider myself a hardcore, they have albums I absolutely love (DSotM, WYWH, Animals, The Wall). Priest took longer. I've seen them a couple times (2005, and then again in 2010 on the British Steel anniversary trek). I knew the hits. But it was just over the past decade or so that I started buying the back catalog and listening. And for my money, I think after repeated listens, the regular version of Invincible Shield is one of the best albums they ever recorded.
Again, I'm not an expert. I have most, but not all, of their albums (I don't have Turbo, Demolition, or Point of Entry yet). But Invincible Shield is just so damn good. Very impressed. Sadly, the only date they are playing remotely near me is a festival, and I'm not a festival guy, so I'm pretty bummed I may not get to see them on this tour.