You should select any experience that lets you do more than simply recalling events. It needs to be something that allows you to reflect on what happened and show self-awareness.
Compare the following two extracts.
I had arranged to meet my friends at ten o'clock. I left the house at half nine and walked to the bus stop. I had to wait twenty minutes because the bus was late. Eventually I got to town.
It was with excitement and some nervousness I set off to meet my friends in town at the agreed time. As luck would have it the bus was late - this day of all days!
Both examples are about the same experience. Yet the second one has a sense of reflection, thinking back over the event. It is not just a list of things that happened.
To a reader the really interesting things are not the big events and every detail of what happened but the writer's own thoughts and feelings about what happened and how they are described.
Consider these two different introductory paragraphs for a reflective essay entitled ‘My Ambition’.
I have always wanted to be an actress. It is my main ambition. Drama is my favourite subject at school and I would like to study it in more depth.
Acting: it has been my sole ambition ever since the day I met David Tennant on a plane coming back from holiday. We sat for hours chatting and planning my next step in my soon to be startling career… Well no, not really, but this is what acting is all about - pretending to be someone else for day, an hour, a while.
The first one is purely factual. The second one uses an anecdote to catch the reader’s interest and then adds a further imaginative twist by saying “No, not really”.