For me it was building my rust=bucket diorama. Specifically because I did not have to care at all about the finish of the truck, or anything like that, because I was essentially destroying it. LOL The other part of the build was that it took me to a realm of ultimate experimentation. I melted the tirea, and molded putty to get the flat look, I peeled at the seat with an exacto knife and stuffed bits of a Q-tip in it to simulate torn seats, I found out Pantene hairspray reacts great with acrylic paint to get the salted/white effect in the rust/paint, I also used rock salt to simulate the tempered glass that's shattered on the dash and glued some to clear parts sheet and stuck in the window frame. And finally used earth tone pastels to get the dusted and weathered look.
How about the rest of you? Does anyone have a personal favorite or "the one" in the build ups that stands out above the rest for you?