For me, Stand-Up. I have a UK "bulls-eye" pressed at Orlake in mint condition that I got before some of these records got pricey.
Thanks to Ian Anderson's generosity, I was able to interview him for my website several years ago, focusing largely on the transition from the early band to the medieval rock ballad style for which Stand-Up is the template. Aqualung also endures as a great record, though it was played to death at the time. I know I did a shoot-out of a bunch of different pressings-- there were at the time some bargains, not sure of the market now. One particular Reprise was very close to the WLP, but I also liked the DCC. And Steve Wilson's remix fixed some issues despite its digital origins.
Old thread. Surprised I hadn't posted to it previously.