Ah, tough one. Each album is killer. I went with The Dream Sequencer, mainly because the trilogy of "My House On Mars," "2084" and "One Small Step" is just mindblowing to me. Those three tracks give me goose bumps every time I hear them! And the rest of the album is just really surreal to me, very progressive, but atmospheric. I can really get into the songs on this album.
I love it because it's the "different" Ayreon album. All the other albums share really obvious similarities, but The Dream Sequencer, to my ears, stands to the side a bit and comes off as a little more unique.