My favorite is Thick as a Brick! Last time I saw Tull, in Ian's own words "a progrock nightmare" in referring to the recording sessions:)
Dream Theater - Images&Words. First prog album I've ever heard. I fell in love with this music. After half a year of listening to I started to slowly understand what they are playing :-) Understand - I've been a heavy metal fan before.
I can't answer this..........
I could not pick one album as the best ever. The various sub -genres of prog make it almost impossible. It woud be like comparing apples to oranges to tomatoes and mangoes.
I'm 54 and like Rock Prog music since long time now discovering this genre of music when I listened Pink Floyd first time when I was 14 and not with their first album but their album Meddle. So I'm still since that time big Fan of Yes, Genesis, Pink Floyd Marillion, Pendragon, Enchant and many many more recent Rock Prog bands and particulary the Neo Prog german band SYLVAN and I got lot of emotions like I rarely had since a long time discovering and listening their concept album POSTHUMOUS SILENCE (2006) and the fantastic voice of their singer Marco Glühmann. Posthumous Silence