This is always a tough one; this thread's cousin, the Golden Age Year in Review thread, was compounded by comics-related items in addition to books, as I picked up my first ever keeper pulp, a Sunday strip OA from 1942 (Smilin' Jack) and the Feb 27, 1940 Look magazine, with an original two-page Superman story where he drags Hitler and Stalin out of their HQ's and brings them before a world court. But in the end its about the comics, and there's 2 Silver Age keys that have to be here, so I am naming an honorable mention as well, as I can't quite bump either of these Silver books for my #2 GA addition this year.
So here's my top 3 and honorable mention, in no particular order: In the case of the OAAW #83, I bought an entire 1,800-book collection to add it; the Action 252 came out of an LCS that rarely has major keys; the Bullseye 11 was buried in an ebay lot of Golden Age (not giveaway priced or even cheap by any means, but this book does not come up often) and the Superman #14 was part of a comic collection that a rare books dealer a couple hours from me had landed. I saw it on ebay but noticed the city and deduced the shop from the handle, then went in-person to see it.