Maverick said:
Trust Mav to try and classify true art into little categories with shiny, bright stars...
Basically, if you want to get a Priest album you have a wealth of styles to choose from. First off, you have their 70's era albums (Rocka Rolla-Hell Bent for Leather). Rocka Rolla is (as expected from a first album) quite dodgy and a bit hit-and-miss, more hard rock than metal. Sad Wings of Destiny is a masterpiece, as are Stained Class and the monumental Unleashed in the East (live). Through this era, you will find a very 70's style of metal in their music; not too heavy, but some great melodies and riffs going on, with Rob Halford's voice high, yet strained. Highly recommended-try Unleashed in the East to get the best coverage.
Next we have the 80's era (British Steel to Turbo). This era is defined by fast paced rockers, a la Maiden. British Steel is their NotB (i.e. a breakthrough album). Point of Entry is commercial 80's metal, if that's your thing. Screaming for Vengeance is pretty powerful almost-power metal. Defenders of the Faith is fast and furious, lovely stuff (esp. Sentinel). You could get 'Priest: Live!' to sum up this era, but I would personally go for either British Steel or Screaming for Vengeance for this period.
Now we reach the 90's era (Ram it Down-present)*. Hooo, boy, NOW we're talking. This is bloody lethal speed metal. Fast riffing, Scott Travis and Rob's vocals higher than most audible frequencies. If you want to get something from this period, BUY PAINKILLER!!!!!! If you like speed metal, you will NOT regret it! Also, try to avoid the Tim Owens era albums (Jugulator and Demolition). They're similarly hit and miss, without the lyrical and vocal benefits Rob brought to the band. I like some of the stuff, but it's not good to get you into them.
Also, try getting Metal Works: 73-93. It covers the band's best bits, while ignoring some of the filler. 35 songs of pure power, right across the board!
So to summarise,
I would recommend these:
Metal Works ('73 - '93) to start
Unleashed in the East
Painkiller
Screaming For Vengeance/ Defenders of the Faith
Stained Class
This will give you a broad summary of what Priest are all about. But, if you like Maiden, you'll love Priest (unless you despise more rock-y metal...)
Keep the faith
Silky
*Yes, I know Ram it Down was released in '88, but it effectively bridged the stylistic gap between Turbo and Painkiller, so I included it here
EDIT: Mav, are you seriously suggesting he use the Commentary scores in judging? The only ones that aren't at least 4.5 stars are Sad Wings, Stained Class, Hell Bent... and Ram it Down, all of which deserve the same!