Okay, here goes. Instead of ranking the albums, I will put them in tiers:
Very good/great albumsCultosaurus Erectus - Song for song, I think this is their best album. It has seven great tunes, one pretty good song (Fallen Angel) and only one clunker (Hungry Boys). Songs like Black Blade, Monsters, Deadline, The Marshall Plan and Lips in the Hills are some of their best tunes over. Great flow, too.
Fire of Unknown Origin - Very similar to the previous one mentioned, only it has no clunkers, but only a couple of songs I would call great. It is consistently very good.
Imaginos - Some get hung up on this not being a true B.O.C. album - because it was written mostly by producer Sandy Pearlman and ex-drummer Albert Bouchard, plus tons of extra musicians are used (including, among others, Joe Satriani and Robby Krieger - but I don't care about that. All I care about is that this album kicks ass, and has five legitimately great songs: Les Invisibles, In the Presence of Another World, The Siege and Investiture of Baron von Frankenstein's Castle at Weisseria, Astronomy (a rework of the original from Secret Treaties) and Blue Oyster Cult (a rework of Subhuman from Secret Treaties). Plus, this is the most epic and adventurous B.O.C. has ever sounded.
Secret Treaties - Considered by many to be their best album. Most of the songs on this album are B.O.C. classics that have been played live by the band non-stop over the last few decades. The production is a tad off-putting - the sound is very clear, but the low end is, well, non-existent. Once you get used to the sound, the greatness of most of the songs is impossible to deny.
Heaven Forbid - Like I said earlier, almost every track is a winner, and this is easily one of their best-sounding records (the rocking songs have balls thanks to the great sound). Great record.
Above average albumsSpectres - The first two and last two songs on this album are worth the price alone; four fantastic tunes. Tracks 3-8 are a mixed bag, with some of them being good and some of them being somewhat average; I like tracks 5-8 quite a bit, while considering 3 and 4 pretty average.
Tyranny and Mutation - Similar in sound to Secret Treaties (them being released a year), this is nearly as good. Every song ranges from good to very good, the majority leaning towards the latter.
The Revolution by Night - Long underrated by many fans, this has three of their best songs ever - Take Me Away, Shooting Shark and Feel the Thunder - and the rest, despite being more melodic and drenched in keyboards leads at times, is surprisingly solid. This album definitely has a bit of an 80s feel, it being from 1983, but don't sleep on this record.
Good albumsAgents of Fortune - Don't Fear the Reaper and E.T.I. are both totally awesome, and the album is worth it just for those two songs (although in this day and age, where we can buy individual songs from iTunes, amazon, etc., I guess that is a moot point). The rest is fairly inconsistent, ranging from good (Morning Final and This Ain't the Summer of Love) to interestingly strange (The Revenge of Vera Gemini and Tattoo Vampire) to just plain awful (True Confessions).
Curse of the Hidden Mirror - Their last studio album, from 2001, has some great songs - Dance on Stilts, The Old Gods Return (which sounds like vintage B.O.C.) and the awesome Stone of Love - but it also has a bunch of forgettable songs. Showtime, One Step Ahead of the Devil, I Just Like to Be Bad and Here Comes the Feeling are all songs I don't hate, but don't care if I ever hear them again.
Blue Φyster Cult - The self-titled debut is good, but the sound of it is very poor, marring the overall good first effort. Think of it as being their When Dream and Day Unite from a sound standpoint, except this was 1972, not 1989.
Do not buy these albums (only buy the songs individually that I recommend)Mirrors - The Great Sun Jester, In Thee and The Vigil are all great. Nothing else is essential or worthy.
Club Ninja - Perfect Water and Madness to the Method are great, Dancin' in the Ruins and White Flags are very enjoyable, and When the War Comes is oddly enjoyable on the right day. The other four songs totally blow.
There ya go.