This is hard for those of us who never really stopped since they were kids, but here's some milestones for me:
BW Deluxe Cheetor, age 4: before Beast wars, I'd just get a select character from a toyline that I liked; Goliath from Gargoyles, the red and Green Ranger, Raphael from TMNT, etc. BW got me wanting to collect everything I coud from that line just because I found the designs so interesting. While the show played a hand in my enjoyment of it, its the first time the toys took precedence for me.
Samurai Spawn: Age 9: Got this as a gift from my brother. First figure to really get me to appreciate sculpting and detailed paint in figures, as well as the articulation on top of that. While I didn't really have the money to buy figures, it got me window shopping for McFarlane stuff at KB and Suncoast/Sam Goody.
Marvel Legends First Appearance Spider-man, Age 15:After Beast Wars I basically stuck to being a TF fan, though my heart wasn't as in it. Beast Machines was bad, and while RiD was cool, Armada was disappointing. Still, I kept on buying TFs primarily because fandom is a hell of a drug, but I did look at Marvel Legends from time to time, and got the super poseable SM2 Spidey from that line. Around this time, I started getting interested in Silver Age comics and Steve Ditko in particular, so when I saw the Sentinel wave Spider-man I had to have it. While it isn't a great figure, really, it got me hooked on Marvel Legends and made me renounce collecting Armada/Energon figures over contemporary Marvel Legends,which seemed such better toys. While I'd continue to collect TFs, too, I'd be more selective about it.
SHF Akibaranger Season 2 Blue and Yellow, Age 22: This is what got me hooked on imports. While I had a few, they were mostly convention purchases and transformers. Hobby Search lowered these to to a mere $2 each(holy shit), I had to bite. I was so impressed, I basically switched to primarily collecting imports for a few years.