With Porcupine Tree you have some great options for where to start. It really depends on what type of rock you prefer. Here's how their albums break down.
The Sky Moves Sideways - "Floyd-ish", psychedelic
Signify - Psychedelic, trance, ambient
Stupid Dream - song writer, pop like, British rock
Lightbulb Sun - British rock, ambient, a tad epic
In Absentia - some hard rock, "Floyd-ish", dark, layered
Personally, there's not a bad cd they've put out, but most fans start with In Absentia and work their way back. They're working on a new cd this year and so far Steven Wilson has used words such as dark, heavy, and long epics to describe the new songs. I can't wait to hear them. Hope you enjoy their music.
Jason
