Interesting questions, Pete... I've thought about this a while (maybe too long) and I cast my vote with the Lennon tribute show for these reasons:
I approached the question concerning a show sent to the Library of Congress to consider which "AT40" program might be considered the "quintessential" AT40, or the most typical.
Well, my personal music preferences aside (I'd go for early to mid 1970s myself), a quintessential AT40 would include all the elements that the show is known for... first off, Casey himself (no guest hosts), teases, chart trivia, at least one question letter (assuming the early QL that was cut out is restored), human interest features and, of course, long-distance dedications, which would leave out almost all the three-hour shows... factor in a very heartfelt tribute to Lennon and you've got a very good example of why we all love "AT40."
By definition, a quintessential AT40 would preclude specials, which were, of course, special but not typical of the show.
As far as the question about owning a master copy... Well, I'd take almost any one from my preferred early to mid-70s period (or, indeed, any of them at all), but I'd go for 12-4-71 or 9-7-74, as those represented, respectively, the first show that I sat down and wrote down all the songs, and then, after not hearing the show for a couple of years as it was not aired in the St. Louis market, in a bit of serendipity, stumbling onto the opening theme from a station in Mount Vernon, Ill.
I'd also like to add something to the discussion that Pete didn't necessarily ask for but has been touched upon in replies.... I'd like to maintain that NOW is the golden age of "American Top 40" for these reasons:
1. Anyone with satellite TV and/or a computer can listen to four different broadcasts of "AT40" each weekend... and record them with relative crystal-clear clarity for posterity.
2. Casey's still doing his thing with two current shows every weekend, and anyone with a computer can listen. No matter that it's not on a local station anymore.
3. I haven't listened to the Ryan Seacrest "AT40" for years, but at least Casey and Don Bustany's brainchild is still around.
4. The books by Pete and Rob Durkee. I know a lot more about the shows I listened to as a kid and young adult now than I ever knew back then.
5. Message boards such as this. For years, I thought I was the only one who was affected by this radio program. Now, I know that there are many more who share, and even surpass, my love for "AT40.'
6. Boards such as RadioInsight that allow others to share programs, and technology such as FTP and file download services that grease the skids for that.
When I first started listening to AT40 in the very early 70s, I could hear only one show a week (albeit with much anticipation). Now, with trading, CD burning, streaming, etc., I can literally listen to AT40 -- any show that I can put my hands on -- any time of the day and night.
That's why I believe that NOW is the golden age of "AT40."
Of course, that's just my opinion. Thanks for allowing me to share it with you.