By order of lasting positive effects,

1. First vape (a cigarette-shaped item from the corner store, cherry flavor). I had been smoking with suicidal enthusiasm for 18 years. I was out of breath, coughing, stinking, and (at $7 a pack) broke. A decade later, I still vape, but I can breathe now.

2. First not-fully-depreciated used car (3-year-old 2012 Focus SEL hatch with 30K mi). Apart from warranty transmission work, the car's been stable, and pretty. The real change was introducing me to finance and lending. I grew up poor with a debt addicted dad. At 32, I had never had a credit card. I've still never given the bastards a dime, but I've pulled in thousands in rewards and have an outstanding credit score.

3. Passport / first international airline ticket. I mean, yeah. I hadn't had any desire to leave my state, let alone see the world. At 20, I grudgingly flew to Europe to visit my girlfriend who was studying abroad. We didn't last, but the travel bug did.

Honorable mention, only because it isn't technically a purchase would be my first union payment. Best deal ever.

Edit: how do you add line breaks? It worked, magically, when I made the list heading bold, but that was hella annoying.