To me, the hardest one was (Spoiler:) The one in the last Bowser level, where you needed to have two clouds stocked from the previous area, but getting to the launch star without using up more than one cloud required some tricky jumping and you still needed to use a cloud to get to it, so there's no room for error.

By the way, (Spoiler:) I got the ones in Flip-Swap using a backflip and spin.