I would say floor. It's not my daughter's favorite event by any means, but it's the one that she (and the rest of her team) consistently scored high at. And the one where most other teams did too. It also seems like it's "easier" to learn higher lever skills on floor compared to other events.
Floor. Lot of different areas to excel in (opposed to vault). If tumbling is not your forte, it is easy to keep the difficult tumbling down and level up on dance elements.
For me personally it was bars. I was a very strong but not very flexible gymnast. Bars was the one apparatus I could really use my strength and my lack of flexibility wasn't really an issue.
But for my gymnasts it is floor. Almost universally my gymnasts are all a significantly higher level on floor than the other apparatus. I think this may just have a lot to do with my own chancing philosophy, as I spend a lot f time developing their tumbling and handstand skills, as I believe this is the core of all their apparatus.