Old Faves:
Jethro Tull - "Heavy Horses", a (mostly) acoustic masterpiece!
Todd Rundgren's "Utopia" (1st album with the eye cover)
King Crimson - Most any, but "Lark's Tongues in Aspic" (with Bill Bruford drumming!) and "Islands" are personal favorites.
New faves:
THE FLOWER KINGS - Don't mind the name, these guys from Sweden are incredible! Yes, you can find references to ELP, YES, Crimson, and Genesis all over the place, but they are also quite original. "Stardust We Are" is possibly one of the top 10 prog rock double albums of all time, and "Rainmaker" is also great, with a little heavier metal emphasis. (I use a track from that album - "Road to Sanctuary" - to demo my system - decently recorded, with well recorded classical and acostic guitars and a vocal in the middle section that comes from the extreme right and waaaay back in the soundstage).
I'll strongly 2nd the PORCUPINE TREE recommendation by Tvad.
KEVIN GILBERT "The Shaming of the True" a truly incredible piece of work, a prog concept masterpiece, worth hunting down. Also see "Toy Matinee" and "Thud"
Enjoy!
James