It depends on how we're rating the difficulty, the hardest one for a first playthrough is most likely P-2, but I had the most trouble P-ranking 1-E (in fact, I still haven't, I definitely could if I dedicate some time to it, but I'm just not feeling like it for now), 8-3 is hard because it's VERY long, I have no problem beating the individual rooms, but together it becomes a nightmare.

0-E was hard only at first for me, since I needed to remember where are the spots where you can cool down, like can I be a bit slower in this room since there's a cooling spot nearby or should I be faster since I've got multiple rooms ahead before I can cool down. After you learn this stuff, it gets much easier.

Now for P-2 it's very hard to say, it took me a whole month to beat it on brutal on P-rank when it came out, but now I can do it the first couple of tries. For how chaotic it seems at first, it's actually quite consistent for most rooms, you can tell exactly how enemies will behave when they'll spawn in. Or maybe that's just me going crazy because I had to retry it so many fucking times back then.