I'm not through yet, (I'm at the forest right now), but my personal nightmare was 4-5. The rocket barrel level with the giant bat. Pure. Horror.

The later rocket barrel levels are pretty tough, although the over all game is pretty easy.

1-K for now was the hardest, 'cause i died sooo many times (used like 50 balloons), if it weren't for it i wouldn't have 500 coins lol . But after you beat it, playing again is easy, it'm more memorizing. The bat level is aaaawful died a lot just to get the lettters and puzzle parts why they have to be so close to the rocks? :@, but it's one heck of a game

OH MY GOD!

i just made level 5-8, where the spiders are chasing you. I've never played such a hard level in my whole life!

...And I actually got all puzzle pieces AND all letters. The fucking "N" was a disaster. I probably wasted around 50 lifes in that mess of a level.

Most difficulty i've run into is stuff that i couldn't figure out. The various challenges i can surmount with effort, but i've pulled my hair out on certain things, like those screamy monsters that stack on top of one another, needed the superguide to show me what i was doing wrong there, and similarly with one barrel level where i couldn't figure out that one barrel would eventually move on its own into the position i needed

I beat it in my first try, and in the second try I got all of the letters.

Anyways, its got to be 4k through 6k in mirror mode, pain in the arse I tell you. 

The shiny golds are easy to get in the mine cart/rocket barrel levels btw.

Well, Cave World shut me up entirely. Beat that world by beating only 2 stages: the first and the boss, and let the great white ape sweep me through the two death traps in between.

Fuck that bat world and whoever designed it, and that cheap ass mine cart stage in between. Anything that encourages no-look premature jumps like that is stupid, and stupidly designed.

Nothing in either of those levels, or anywhere in the game that I recall, asks you to leap before you look.  What the game does do, at several points, is demand that you pay close attention to what's ahead of you, and to not panic: you may only be given a half-second warning that the tracks ahead of you have dropped a few feet, but if you kept your cool and waited that's plenty of time. 

Basically, what I'm saying is that DKCR tests your reflexes and patience much more than most modern games.  But if you stay calm and ready, you'll be golden.

There were two jumps in there where you had to jump before you saw the end of the tracks, and even then there was a decent shot of you overshooting (unless there's a method of arresting momentum midjump that i'm unaware of). The second of those jumps was my wall, where i realized i could either let the game do it for me, or snap the disc in half

The bat i at least understood, but those no-look jumps are the end of it.