The downward spiral was certainly a highlight for Trent. WIth Teeth seems to be where a lot of my friends started listening to him though. Overall, you should listen to all the albums because they each offer something new.
I (still, after everything) believe in Billy Corgan.
I personally loved The Fragile more than anything else, but still, you really can't go wrong with any of their albums. Although, I'd say The Downward Spiral is probably the best place to start.
Here's my list

1 - Ghosts
2 - The Downward Spiral
3 - The Fragile
4 - Broken
5 - Pretty Hate Machine
6 - Year Zero
7 - The Slip
8 - With Teeth

If you liked the slip you are definitely going to love their other stuff.

Actually, I will do it the proper way and listen to their albums in chronological order.

Remember to listen to Broken, it's an EP but it's still great
Last edited by Duffman123 at Sep 4, 2010,
I started when my uncle burned me a copy of Broken, so maybe start there. Also, maybe ask the NIN thread.
Quote by GezzyDiversion
Listened to all of The Slip, Downward Spiral and Fragile.

Currently listening to Ghosts.

Ghosts took me about 4 months to love so good luck
Go download ' And All That Could Have Been' its one of their live albums
Best live album I have ever heard from any band NIN kick ass live
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Quote by Duffman123
Ghosts took me about 4 months to love so good luck

I loved it right away, but you probably have to appreciate ambient music to do that.