For me it started with the 1st series, so Gadzooka is one of my all-time favorites. But my 1st series stickers got destroyed, so they were forgotten after shedding a few tears and collecting the 2nd and 3rd, before the craze hit. I remember trading the 4th and 5th in school, only getting a Windhex that someone had stuck on their notebook, and never seeing a Choke Wagon. I remember my sister helping me get the entire 6th series in one day by probably only spending $2 - two trips of 19 packs + tax. I vaguely remember the 7th-10th and multiple trips to the 7-11 to look for them whenever my parents would take me. I don't think I ever saw the 11th, and I'm still not so fond of the magazines. The 12th showed up at the concession stand where we played baseball, so I remember a resurgence of my collecting during the summer. After that I don't remember seeing them anymore.
So the 4th, 5th, and 6th probably have the fondest memories for me, but my favorite sticker is Wormy Packages, and my favorite puzzle is definitely Gadzooka.