Funny, I just re-read this old thread because the headline sucked me in and I thought, "No brainer #1 even though Avengers #4 is my favorite book. What am I missing?" I still think the majority will stick with #1, but I see the argument that the real Avengers begins with Cap. Someone said without Avengers 1 there's no Cap revival, but it could be argued that the title might not have lasted had they not come up with #4. So it could be viewed a little like Hulk 180 and 181, though reality is it won't.
I do think Avengers 4 and Flash 123 big as they are are still the most underrated SA keys for basically introducing the concept of linkage to the GA. I do have to wonder though, thanks to this thread, what the values would look like if the first and fourth Avengers stories as we know them had appeared not in a series starting with #1 but a converted title a la TOS 39 or JIM 83. Fun stuff to think about.
i quoted the above because my 70's and 80's recollection had TTA27 in a bigger weight class than DD1 and ST110, due to relative difficulty to locate. Before the Internet era, that was a legendarily tough early SA book to locate. The other Marvel bottom-feeder key as I remember it was Sgt Fury #1.