OK, this is my opinion....and of course we all entitled to them!!
Crosses next to the NO - NOs.......!!!
Arena-Contagion
- Arena have definitley improved, so these top 2 I would go for..
Arena-Pepper’s Ghost
Arena-The Visitor - OK.....
Arena-Songs from the lion cage - X
Arena-Pride - X
Emerson Lake and Palmer-Emerson Lake and Palmer - X _ I like ELP, but this CD is not a neccesary purchase.
Emerson Lake and Palmer-Brain Salad Surgery - 
Emerson Lake and Palmer-Trilogy -
Emerson Lake and Palmer-Tarkus -
Fish-Vigil in a Wilderness of Mirrors - OK album..dont start here...
Fish-Sunsets on Empire 
Fish-Raingods with Zippos 
Fish-Field of Crows - X - not necessary.
The Flower Kings-Retropolis - Good album, but dont start here -go to>
The Flower Kings-Stardust we are 
-their best effort, I think
The Flower Kings-Space Revolver 
The Flower Kings-Unfold the Future
Genesis-The Lamb Lies Down on Broadway 
- absolutley necessary
Gentle Giant-Free Hand -X -I would avoid GG with a sharp stick, but many like them.
Gentle Giant- Gentle Giant-X
Gentle Giant-Octopus -X
Gentle Giant-The Power and The Glory -X
Gentle Giant-Three Friends-X
Glass Hammer-Chronometree -not necessary, not the placer to start
Glass Hammer-Lex Rex 
Glass Hammer-Shadowlands
IQ- The Wake
-NECESSARY -cant really go that wrong w/IQ, except the Paul Menel CDs are poppy, but still are decent enough song wise..
IQ- The Seventh House 
IQ- Subterranea 
Jethro Tull-Heavy Horses 
Jethro Tull-Minstrel in the Gallery 
Jethro Tull-WarChild -Not the greatest Tull CD, there are MANY better before this, such as 'Thick as a brick', 'songs from the wood' and 'Broadsword and the beast'
King Crimson-In the Court of the Crimson King - X -NoWAY is this as necessary as many here would have you believe!
King Crimson-Red
Magma-Kobaïa-X -Magma are the most AQUIRED taste in prog, test yourself on MDK first see if you can deal with it, then proceed.
Magma-Üdü Wüdü-
Magma-Retrospektiw Vol. 3 -X
Magma-Mekanik Destruktiw Kommandoh -X
Magma-1001° Centigrades -X
Magma-Attahk-X
Magma-Köhntarkösz -X
The Mars Volta-De-loused in the Comatorium -X yuk, most overrated band here, except for Opeth maybe..
Neal Morse-It's Not Too Late -OK, album, NOT religious, but not proggy either, short songs, some are v.poppy, but all are decent, especially the closing song.
Neal Morse-One -
Less Religion, and proggy, OK album
Neal Morse-Neal Morse
-good album, some prog, some pop, but all good..
Neal Morse-Testimony XX -yuk, religious overblown/overlong nonsense, avoid like plague
Mike Oldfield-Tubular Bells -
essential prog
Opeth-Blackwater Park -x
Opeth-Damnationx- OK album, has no Growling vox, so you can hear how good they COULD be, what a shame the vocals are so silly on their other albums, a waste.
Pendragon-The Masquerade Overture - OK, prefer 'The world'
Pendragon-Not of this world -as above, but get 'Masquerade..' before this.
Pink Floyd-Animals 
-ABSOLUTLEY essential album
Pink Floyd-Dark Side of the Moon
-almost as essential as 'Animals'
Rick Wakeman-The Six Wives Of Henry VIII
-can't go wrong w/these 3..
Rick Wakeman-Journey to The centre of the earth 
Rick Wakeman-The Myths and Legends of King Arthur and the Knights of the Round Table 
Spock’s Beard-The Light 
-essential 90s prog album
Spock’s Beard-Snow -overlong ..good, but a missed opportunity.
Spock’s Beard- The Kindness of Strangers 
Spock’s Beard-Beware of Darkness 
Transatlantic-Stolt Morse Portnoy Trewevas (Smpte) 
-Great album
Transatlantic-Bridge Across Forever -
good, but not as good as the first.
Van der Graaf Generator-Godbluff-x -VDGG -another aquired taste, check out one of the two below and check whether you can deal with Hammil's voice first!
Van der Graaf Generator-Pawn Hearts
Van der Graaf Generator-H to He, Who Am the Only One
Yes-Close to the Edge
-essential
Yes-Relayer 
MORE essential than above, for me
Yes-Tales from Topographic Oceans
great, but BE prepared, it is a tough one to crack!
Yes-Drama
good, but no Anderson, songs are decent though
Yes-Going for The One 
-another ESSENTIAL album..