I loooveeee Porcupine Tree
I've got Deadwing, On The Sunday of Life and Signify
Deadwing is a great commercial album, every track is very listenable. The latter 2 however are much more progressive and experimental.
I think In Absentia is my favourite album though, Blackest Eyes and Trains are awesome, i'm definately getting that album at some point.
EDIT:
Tommy get the special edition of Signify, it's 2 discs and it's all very experimental
Waiting Part One and Two are 2 of my favourites on there.