I don’t think I can specify my best idea.
Once upon a time I saw a NaNoWriMo poster. I decided to do it. The novel wasn’t well-written, but it was fun.
Another time I decided to carry on studying German. I was good at it, and it made for a good minor at university. True, I can’t remember much of it, but I imagine if I were to be immersed in the language on a day-by-day basis, I’d pick it up and improve the base of what I do know.
Or, there was the time I decided to not make things easy upon myself and not do things the way the rest of my classmates did. I refrained from music and TV and books on the basis of their popularity, meaning that if I was to be entertained I’d have to work harder to find something I truly liked. And it’s been working. Lately I’ve been hunting new music, new bands and wishing I’d known them sooner. And it’s fun not following the pack.
Maybe that was my best idea. Check back in five years though, I bet I’ll have a different response.