Honestly, for me, I don't mind which stage I play on as long as it can be used for competitive play.
With that in mind, I would say BattleField. The stage isn't too big, and you can't run right to get blasted off unlike other flat stages with no borders. Plus, I like how there are platforms which means that you can very easily dodge the other players attacks by moving from platform to platform, though not too much platforms are there so you can't abuse it.