The hardest level I designed? It's definitely Beanbean Cave in Mario's Strange Quest. Here are the details on this stage, for those of you that haven't played this level yet:
As soon as the level starts, you'll notice a red Yoshi to the left, that says he ate way too many cans of Eaty Petey's Meaty Beans. The Yoshi starts farting up a storm, so Mario must run away from the fart cloud for the entire level. What makes this level difficult is the fact that I used the very fast autoscroll for this level. At one point early in the level, the screen moves so fast, that Mario starts getting pushed to the left side of the screen, where the fart cloud is. Every time he touches the fart cloud, he gets hurt by it, so it's a battle to stay away from the left side of the screen as much as possible. The only way to survive is to use the arrow blocks, which launch Mario in the direction they are pointing. Also, in addition to the fart cloud, the other hazards are the spikes and the Beanies. Overall, the level isn't very long (it takes only about 20 seconds to beat), yet it is probably the hardest level I've made.
Ironically, I have an easy time with this level, yet everyone else who has played this level has a very difficult time trying to beat it, even with savestates and slowdown.
Here is a video of me playing through this level flawlessly.
(Note: This is a regular playthrough for me of this level, it's not a TAS).
My Hacks:
Mario's Strange Quest V1.6
Yoshi's Strange Quest V1.3 /
V1.3.1 Beta 4.6
Mario & Yoshi's Strange Quests (2/10/2023 Build)
Other stuff:
My SMW/SMAS/SMAS+W disassembly
Yoshifanatic's Discord Server: A place for fans of my stuff and/or Yoshi to chat with others.