Over the years, I've made so many purchases so it's really hard to say what is my favorite.
Though, I do have some things that lead me to getting a favorite item.
Not really a purchase, but I had to purhcase items to unlock the Ronin head from Aurora at one point.
It was well worth it, and is really one of my favorite.
Actual favorite Home purchase would be my Loco Roco earrings. I never go without them on, and rarely wear any type of other earrings.