Wish I bought my first duplex back in 2010, when the economy had bottomed out, instead of waiting until 2018. Outside of a better degree (I also wish I continued with a double major in CIS/Finance instead of just 1 major in CIS), I don't think there is any single better financial decision I've made. Buying one in 2010 would have been when houses had bottomed out and interest rates were in the 3's. The home today would have been double in price, maybe more.. but more importantly, it would have started me on a path to financial freedom that much sooner. Maybe would have owned 3-4 homes by now instead of just 1.
Definitely wish I was a little smarter with money in my early 20's, overall. That's something I would tell anyone at that age: make the right decision in your 20's and you could be set for life.