The Star World castle.

SPOILERSIf the harder Koopaling chase scenes weren't bad enough, Dry Bowser is the real final boss, and he's faster than his living counterpart at that. The platforms are smaller than the one in the final boss battle as well, but I've already lost many 1-ups and have seen the white leaf block for a while.END OF SPOILERS

I actually just beat him and found him easier, the reason, because you sit on a platform for a while and he goes for that one you just have to have the raccoon suit and jump when he get ready to hit

I must be rushing, then, so I'll take my time when I try again. It's best to try a level again after a while and you've cooled down, rather than keep trying again and again only to get more and more careless.

I'll admit the first part before the boss is hard I die so much, but I beat dry bowser on the 1st try

I've got to say the World Star cannon. That level is so intense, but it is well worth it considering where you go.

I didn't really think the game was hard :/ but the one that gave me the most trouble collecting star coins was bowsers castle.

The Meteor Level in world 6

Gosh the Cannon levels are really hard... Even the World 1 one gave me some trouble. Best thing to do is just stock up on leaves and fly as much as you can, landing on the ground at all fully charges your flight.

The game was, simple no problem. I do have some trouble in some of the coin rush maps like in 1-A, it's the best map for coin rush first level but if I mess up I usually end up losing gold mario meaning I have to start over. Also why can't you use luigi in coin rush mode. You can use him in story mode...?

Probably either the Star World castle or the annoying cannon levels in the secret worlds. I think the Flower world cannon level was a complete nightmare.

For the former found the boss fairly easy, it's just the bit where the Koopalings try and turn Mario into stone requires quite skilled dodging when on a moving platform. And the latter... well, annoying enemies, no way to stop... all the things I hate most.

Finding the secret exit in the ghost house of Mushroom World. I got to that level in August and spent forever looking for that secret exit. I finally gave up. Today, almost three months later, I went back and DUHHHHHHHH ... there it was the first time I went through the level. Sometimes you just have to erase your memory on something and start over.