Jethro Tull - what albums?

You cant deny the greatness of Aqualung though. Roots to Branches is also very good, and definately get Songs From the Wood. Those are my favs out of the few I have.
 
I just got Thick As A Brick and I quite like it. What should be my next purchase?

Aqualung is a classic, but not my favorite Tull record (still, definitely check it out).

I'd suggest the following:
Minstrel in the Gallery
Heavy Horses
Songs From the Wood
War Child
A Passion Play
(similar to Thick As a Brick; one long two-track record, and a concept album; however, it's much harder to get into than Thick As a Brick)
Stormwatch
Broadsword and the Beast

The final two are more 80s than the rest, but still decent records.