If I had to pick just one, it would probably be this one.
1977 I moved to a new town, and asking kids about baseball cards got both odd looks - since I was a high school freshman- and the info that just in the center of town there was a whole store for cards.
Oh the things they had!
Up till then, the only idea I had about cards older than 1969 came from the Hank Aaron specials from 74, and one 1968 I'd traded for.
I really liked the T206s, which they'd put in pockets on 3x5 cards with the players name and some stats. I liked the sort of pastel images and scenery, and would stop by pretty much every chance I had to hang out and talk cards, or just spend a bunch of time looking. But the nice ones were just a bit more than I was willing to spend. a whole 1.50 !
One day the said hello, and by the way we've got something we saved for you. A T206! Not in great shape, in fact almost torn all the way across the upper left corner. But only 20 cents, and it was a Polar bear back. After I showed it off to mom and dad I had a little less trouble getting some extra spending money and eventually would buy more.
So I suppose my first T206 is still my favorite, because it's the card that started me collecting prewar cards. (The file cabinet of 60's-70's commons did the same for the then more modern stuff)