Ok so here we go...
1. The Downward Spiral
2. Year Zero
3. With Teeth
4. The Slip
5. The Fragile
6. Broken
7. Ghosts I-IV
8. Pretty Hate Machine
The Downward Spiral is perfect, everytime a new album comes out I listen to it again just to see if I have overrated it, but I haven't.
I think Year Zero works extremely well conceptually. I get a bit tired somewhere around The Greater Good/Meet Your Master but otherwise I love it.
With Teeth is a good album with some pretty shitty lyrics in places. I actually liked the NIN-Rock sound with Dave Grohl on the drums. I really like listening to the whole thing from start to end. The Line Begins To Blur is actually one of my all time favourites. All The Love in The World, The Hand That Feeds, Right Beside you in Time, Right Where it Belongs and even With Teeth all still are excellent to me.
In comparison, The Fragile has brilliant bits, but the lyrics are just as shitty as WT. I find the whole thing to be a bit unweildy these days. Maybe I'll learn to love it again. It does make excellent trailer music though

I've slipped 'The Slip' in between the two since it still doesn't feel totally album like to me. I think it's actually comparable to Broken. Both feel short and have a few great songs on it. As others have mentioned, it's still early days.
Ghosts is hard to fit in. I really like it a lot and I'm glad Trent made it, but it also feels less than a true NIN album.
And poor old PHM. I like it, but I nearly always like the live versions of these songs (from And All That Could Have been) more than the originals.
(And I have Quake and it's frankly not very listenable except as background music.... to killing things!)