Mine was buying SoulSilver after stopping playing 3rd Gen Pokemon. It's great returning back to the world of Pokemon and I felt like a young child all over again. It led to me discovering wonderful Smogon and Pokemon Youtubers. I've bonded with my brother (he got the other version) through Pokemon and I recently discovered that my very good friend is a Pokemon fan too.
In fact, the whole Pokemon experience is my best Pokemon experience. However, if I had to choose one part, it's me choosing to play Pokemon again after quitting it. I never regret that decision.
P.S. One hundred words was not enough for me to fully talk about the best Pokemon experience but I managed to keep to the limit. :P