Although I hate "labels", Priest were definitely pre-NWOBHM.
I don't think there really is one definitive NWOBHM album, but I'd nominate:
Metal for Muthas, even though a lot of it isn't very good, because it's a rare instance of a compilation album put together near the start of a movement, by people who were actually involved.
Iron Maiden - Iron Maiden because it's the first album, Killers because it's the first with Adrian Smith.
Diamond Head - the White Label album, for cult status, Metallica influence and all-around excellence.
Saxon - Wheels of Steel. Never the best NWOBHM band, but in some ways the definitive one for me - a bunch of ugly British blokes with loud guitars and lyrics about motorbikes, leather and, er, Metal.
Witchfinder General - Death Penalty for that ridiculous album cover.
Anything on Neat Records, the NWOBHM label. Tygers Of Pan Tang, Raven, Fist, White Spirit...
Special mentions for Praying Mantis and Trespass for being really good but never really making it....