For me it's probably:
Deliverance
Blackwater Park
Still Life
My Arms, Your Hearse
Damnation
Morningrise
Orchid
Ghost Reveries
Watershed
Just because Ghost Reveries and Watershed are at the bottom of my list doesn't mean I dislike them. I actually like them both very much, just not as much as what came before. Ghost Reveries, in particular, is an eff-ing CRACKING album. I just don't get the same buzz off of that as I do the others, like Still Life, Blackwater and Deliverance.
In addition to the regular releases, I've picked up the deluxe editions of Blackwater Park, Still Life, Ghost Reveries and Watershed just for the 5.1 mixes. Blackwater Park KILLS in 5.1, but I haven't had the time to dedicate to the others yet.
(edit for sloppy spelling!)