Stand Up is the way to go, my favorite Tull album next to Thick As A Brick. Agree with what Laura said, Benefit is also a must. On Stand Up, "We Used To Know," while not a particularly popular track is my absolute favorite. It is a song that has no chorus, and the only thing that releases the building tension of NOT having a chorus is an absolutely blistering, distorted guitar solo. And the strumming guitar propels the song on and on, a never-ending frenzy. This is magic prog at its most majestic.
Unfortunately, like the Something Else through Muswell Hillbillies Kinks, it's a bit hard to find either record in good shape. You'll luck out if you find the "stand up" insert in the gatefold Stand Up album.
Paul