See first video and world 4 part 1 for all details.
0:01 Fetid Swim (secret): The secret exit requires you to get a key on the right side, and bring it to the left, although it is much easier, though more time-consuming to spin jump across the gap without the key, hit a P-Switch to make a temporary bridge, then being the key back to the left with you.
2:51 Heavy Artillery: This level is very hard. You'll have to deal with both slippery ground and precise jumps, while bullets keep coming at you. However, this level is one of the spots in the game where you can farm infinite lives.
6:24 #6 Frozen Outpost: This has to be by far the hardest level in World 6. What makes it so hard is the vertical section in the second half. There are bullets, spikes, and other hazards to dodge while making jumps across very narrow, slippery platforms. One of the hardest parts of that whole level has to be 9:15. This level is just simply intense. At least the rest of the level after that shaft isn't too bad.
levelengine presently has 668,534 views for Super Mario World across 1,547 videos, accounting for over 28 days of watchable video on his channel. This is 18.25% of the total watchable video for Super Mario World on levelengine's YouTube channel.
1