For me, it's Super Mario Bros 2 USA. Not Lost Levels, Mario 2 USA.
I actually made a video about how much I despise Mario 2 USA here [ https://odysee.com/@Protoman00:1/Bubsy-1-Is-Better-Than-Super-Mario-Bros-2-USA:c ] but I'll summarize. I struggled to deal with the game's slippery controls, regardless of what character I used. Every character in the game sucked, including Toad cause that man CAN'T JUMP. Also the level design and enemy placement is just plain bad a lot of the time. Add all these frustrations together and this game was so hard for me. It's one of the few games that made me actually angry, which rarely happens to me. It didn't help that I hated the music in the game to.
The hardest worlds in the game were the desert world and the ice world. Straight pain. When I beat Mario 2 USA, I was so shocked that it ended so quickly.
So what is y'all's hardest game you've beaten?
Side Note: Before I tried to beat Mario 2, I actually used to regard Mega Man X has the hardest game I've beat because of it's final boss.