Rain
So I figured you know a thing or two about them :P
LOL, some would say that Krist
Rain
... and based on your list I just picked Painkiller. I guess that'll leave Angel of Retribution and Ram it Down for the next purchases.
Painkiller is a great choice. In my opinion, it's almost the perfect metal record, and it still stands up today - to me that's always a sign of a classic album. I think what
really makes it stand out is though is that it's the first record they made with Scott Travis on drums. He is just so much better than Dave Holland, and it really shows in contrast to the albums Holland played on.
Ram It Down is a cracking album too, but more of 'its time' than Painkiller. Although it's dated better than the synth-laden stuff on
Turbo, it still screams "Hair Metal" - albeit still sounding like a Priest album. Check out the awesome guitar solo in the title track -
Ram It Down 
Incidentally, for the Epitaph tour, they chose
Blood Red Skies as the track from this album they included in the set.
I have to agree with you completely about
Johnny B Goode - that's up (down?) there, along with Metallica's massacre of (Thin Lizzy's version of)
Whiskey In The Jar, as one of the worst cover songs ever

Incidentally, if you want to hear a truly wonderful cover of a Thin Lizzy song, check out Megadeth's awesome version of
Cold Sweat from their new CD -
Super Collider.
As far as
Angel Of Retribution goes, I think it was always going to be special as the first album the band recorded after Rob Halford rejoined the band nearly 15 years after he left to pursue his solo career. It certainly doesn't disappoint, and with Halford back on the team, the quality of the songwriting is noticeably of a much higher quality than the rather one-dimensional tracks penned by Downing and Tipton on the two albums (Jugulator & Demolition) they recorded without him.
Standouts for me on Angel Of Retribution are the autobiographical
Deal With The Devil, the hauntingly beautiful
Angel, and
Worth Fighting For.
As if to emphasize the band's reunion and link it to its past, there are a lot of references to titles from their back catalogue in the lyrics of most of the songs.
Rain
I see Nostradamus didn't make it into your list - any particular reason?
Don't get me wrong Krist, it's a very good (double) album, but even as a concept, for me it's just too long. Every 'real' song is joined to the next by orchestrated 'fillers'. It works, but I think they could have trimmed all the fat away and comfortably fitted all the good stuff onto one disc.
Rain
I'd have a tough time making my own top 10. Sad Wings holds a very special place for me, even if I didn't hear it until years later.
I still have the LP
By the way, a new remastered CD version of the album was released a year or so back, and whoever did it made a great job. It's really faithful to the LP, as opposed to the previous CD release which Gull records released without the band's approval to try and cash in. A recent accompanying re-release of
Rocka Rolla is also available. In either case, I don't know if either of these updated remasters were sanctioned or not, although it's not Gull who published them.
Rain
Screaming and Defender I just can't get tired of these days. The material on Defender has the edge but Screaming still has an awesome bunch of songs and I prefer the production. Stained Class, Sin After Sin and Killing Machine would all be pretty close too. And British Steel, well, what can I say... I think Grinder must be one of the first Priest songs I learned - right after the obvious Breaking the Law, and, for some strange reason, a very simplified version of Desert Plains.
Me and my old pal Sid Strummer (bass and vocals) have been jamming Priest songs for over 30 years now! Not as regular as when we were younger, but we have nicer gear now

No doubt you recall the pic of me I recently posted of me posing with the black LP copy
Here's a photocopy of the Priest section of our (extremely tatty) 'set list':
Sadly, although I can hack my way through these on rhythm guitar, my technical ability (or rather. lack of it

) means that I come up a bit short in the solo spots. Still, it's great fun when we get the chance.
If you're up for a few more suggestions, I'd heartily recommend you get yourself Halford's
Resurrection. If anything, it's a little heavier than Priest, but it's a great record which hasn't got a duff song on it. One of my favourite tracks of all time is the wonderfully menacing
Twist from this very album - written, coincidentally, not by Rob, but by Bob Halligan Jnr, who also wrote
Some Heads Are Gonna Roll from Defenders of The Faith!
Also worth a listen is Halford's double live CD
Live Insurrection - as well as a 'best of' selection from his 'Halford' albums and from his previous incarnation as 'Fight', there's also a slew of Priest classics on there as well (including the wonderful
Tyrant, sadly lacking from JP set-lists since
Unleashed In The East).
Finally, and worth getting for the guitar work alone (as well as for Billy Sheehan's bass work on some of the tracks), is Glenn Tipton's first solo project
Baptizm Of Fire - check out the awesome mostly-instrumental title track
HERE (with Sheehan on bass). His singing is pretty dire in places on this album I must admit, but it doesn't detract too much from the whole. I think Tipton's guitar work has been criminally under-rated over the years, I suppose it's a common denominator type association with the much technically inferior KK that's seen him overlooked individually.
It will be interesting to see what the upcoming Priest album will be like, especially now Downing has been replaced by 'young' Ritchie Faulkner - his talent is more akin to Glenn's, and they've said he is involved with the songwriting, so it should be worth waiting for.