Alright, I thought about it. It's easy to lean towards Bats 227 and 232, but when I ask myself what books have stuck with me over the years (even when I wasn't collecting), I find myself actually gravitating towards some of the Kaluta books. Here is a prime example. This 8.5 was originally bought as a filler, but it turned out so nice (the scan is crappy), I decided to keep it. Actually looks nicer than some 9.4s I have.
Tec' 424 deserves honorable mention as well. Another 8.5 that's so nice, I'm not going to bother upgrading from it.
Like many of the Adams covers, I thought these were way ahead of their time, and certainly way ahead of the early BA writing style.