I highly agree with this. I never understood why Gamefreak removed this feature (I mean there was a reason for the slots).The only shame is that they removed the option to buy coins.
Anyways, my answer would probably be the slots. Don't get me wrong, I like Voltorb Flip; however, it does tend to get frustrating at times and there are times when I just don't have the patience for this mini-game. The slots in one way or another always allowed me to get a decent amount of coins by the end of it in the older games.