The Fragile is far and away his best musical offering. I like Ghosts a lot too, makes for awesome background music whilst studying and such.
The Slip, however, is shit. I'm a lifelong NiN fan, I've been listening to them since I was 13, and I've seen them live 9 or 10 times, but unlike many fans like me, I'm not blinded by my love for Trent. The Slip is the sorriest excuse for a musical release I have ever heard by a band I love. Anyone who does not realize it is just a pile of shit that did not make Ghosts or Year Zero is kidding themselves. I love the release style and the statement that it makes, but that record was worth as much as we paid for it, nothing.
Every album is great in its own way. PHM, Broken, TDS, TF, WT, YZ, Ghosts all are amazing, and none of them have any weakness that would prevent me from enjoying them. But as far as I am concerned, The Slip is an embarrassment.