Crud. Several:
87/88 OPC - This is the year I really remember busting packs and chewing gum. I still have a good number of the bigger cards from this set, albeit I had to upgrade almost all of the significant cards. This was the last year of Gretzky the Oiler, and everyone in my school labeled him a traitor that summer (and coincidentally, a LOT of LA Kings caps were on the playground the following year). Looking back at the RC's (which meant zilch at the time) Robitaille, Oates, Vernon, Hextall, Damphousse, Richer, etc. were all fairly big names in the late 80's/early 90's.
90/91 Upper Deck - Suddenly collecting became a serious enterprise. People were actually paying MONEY for cards! Trades were no longer 1 for 1, regardless of player. I couldn't afford OPC premier, so UD became my jam, until it got too expensive as well. Even without Lindros and Belfour you could make an all-star team out of the RC's in this set.
98/99 ITG Be A Player: I was out of the hobby by 94, busy buying up punk and grunge CD's. By the time I left high school the hobby bug bit me once again. Tried 2 packs of this stuff thinking the concept was neat: Joe Thornton and Luc Robitaille gold ended up being my graphs, and I was back in the hobby.... at least until I had to be an adult full-time. At the time the Mats Sundin jersey I pulled from series 2 of this stuff was the biggest card I've ever pulled.
01/02 Fleer GOTG Auto's. Clean design, and a project that took me a few years to complete. Funny enough, when it first came out I only bought half a box or so, took my Tiger Williams and Bobby Smith auto's and called it a day. 7 years later I'd be waiting for the mail truck to deliver the Paul Henderson graph I needed to complete this set.
03/04 ITG Original Six. Where to begin: ITG was on absolute fire at this point. The Original Six line is probably the best team-dedicated product to ever hit the market. (ITG Franchises Canadian being the runner up). Fairly good value (unless you got one of those darned shooters jersey cards.... unless it was in the Rangers box....). For a brief period we all became kids again with this stuff, putting together sets of our favourite Original 6 teams. For whatever reason I did best with the Blackhawks boxes.