Aaron mentioned a lot I would have mentioned, however I'm sure there's still room for moar
Some moar essential classic Prog,
Camel - First four albums
Caravan - If I could Do It All Over Again, In The Land Of Grey & Pink and For girls who grow plump in the Night
Gentle Giant - First 6 albums
Jethro Tull - Thick As A Brick, Aqualung, Song From The Woods
Gong - Angels Egg and You
Soft Machine - Third
Hatfield & The North - Both albums
Focus - Moving Waves and Hamburger Concerto
Supersister - Present For Nancy
Bubu - Anabelas
Comus - First Utterance
No Prog Metal fan could do without
Fates Warning - Awaken The Guardian and The Spectre Within
Queenstyche - The Warning and Operation Mindcrime
Crimson Glory - Crimson Glory and Transcedence
Mekong Delta - The Music Of Eric Zann
Heir Apparent - Graceful Inheritance
Some of the heavier side of 70's prog besides Rush,
Atomic Rooster - Death Walks Behind You
Captain Beyond - Captain Beyond
Groundhogs - Split