Here it is, the essential Cooper. I'll list what ones are worth getting (IMO), what ones you can probably wait for and the ones you only should bother with if you like most of the other stuff.
Essential
Killer (the absolute pinnacle of the group and maybe Coop's career. Essential)
Love It To Death (see below)
Billion Dollar Babies (not as good as Killer but is anything?)
Welcome To My Nightmare (you know this one, nothing more to say)
From The Inside (about his stay in an asylum and close to a concept album, witty and has Bernie Taupin co-writing as well as Bob Ezrin somewhere)
DaDa (be warned, this one is weird, like nothing you've ever heard before and maybe an acquired taste)
Raise Your Fist And Yell (even more dated-sounding than Constrictor but better than Junk. It has Kane Roberts!!!!)
The Last Temptation (religious story about Steven continues 19-something years on)
Brutal Planet (political, religious rock that borders on industrial. Essential)
The Eyes Of Alice Cooper (predecessor to DD and pretty much the same but polished)
Dirty Diamonds (awesome-sounding 70s-like garage rock with a couple of overdubs)
The good, but not essential ones
Hey Stoopid (more 90s-sounding rock that sounds like an MTV-catering exercise. Desmond Child is involved and I think Dianne Warren too
)
School's Out (yeah, it has School's Out. I'll give it that credit)
Muscle Of Love (the last group album)
Goes To Hell (a nice few ballads and nice few rockers but something missing)
Flush The Fashion (good New-Wave but if you don't like NW you mightn't like it. YMMV)
Constrictor (80s hair metal and it shows - fun to be had)
Dragontown (even more political than he'd ever done before)
Along Came A Spider (the latest and a great example of phoning it in)
The ones you should only bother with if you're seriously liking the rest
Lace And Whiskey (horrid in every sense of the word. Maybe even more so)
Special Forces (hmmmmmm...get it for yourself and find out, but only if you don't mind being disappointed)
Zipper Catches Skin (New-Wave at its finest)
Trash (not the worst but bad enough)