The "Signify" album is the transitional release, that was the first album where Steve assembled an actual band. Before that, PT was more of a Wilson side-project.
The pre-Signify albums are very Floyd-ish, but a little more pop-oriented.
Signify thru In Absentia are very song oriented albums, lovely balance of light and heavy.
Deadwing was where the darkness/heaviness started to really creep in.
FOABP and The Incident are kind of a combination of everything, Steven returning to longer song forms and more "progressive" structures. Those albums really need to be listened to all the way through to appreciate. It also helps to listen to Pink Floyd's Animals right before playing The Incident, because Steve does a beautiful job of de-constructing that album for bits and pieces of The Incident.
There have also been numerous EP's. Nil Recurring is still available and should be purchased by anyone who likes PT, it was recorded at the same time as Fear Of A Blank Planet.