That's a good question. I would start with either of thier newest albums, since they are easy to find. I personally would try In Absentia first. It was their 2002 release. That album is a little more accessible. But it still has the vintage PT and all the vocal harmonies and solos aplenty.
If you're liking that, I would actually search the net and purchase their 1996 album Signify before their latest effort Deadwing. It's pretty easy to find for a decent price from Amazon.com, Towerrecords.com, etc. This album, while still having a lot of the qualities of the In Absentia album, still comes out being so totally different at the same time. Only a band as genius as PT can do this.
Then I would move to the new album Deadwing. It features pretty much everything PT has done to date rolled into one album.
Their albums in the middle, Stupid Dream and Lightbulb Sun, are considered their best by many (along with In Absentia, Deadwing, and even Signify, lol). These two albums are out of print right now though. Lightbulb Sun you can get used for a relativly decent price, but Stupid Dream is going for crazy amounts. So, I would wait on finding these albums until a bit later. Plus they are supposed to be remastered and reissued later.
Their even earlier albums are also good, but everyone's tastes are different and these albums are a bit more genre specific. The Sky Moves Sideways is a great album, but it is fairly heavy on the psychedelic, instrumental, experimentation, prog movement.
Plus their are other great albums to get, like Warzsawa, a live album from Poland. It contains mainly songs from the Stupid Dream/Lightbulb Sun era and may be good for a farily early purchase. The Coma Divine live album is very good too.
Basically, all thier albums are great!
