I hate PD15. For some reason, I'm always able to die there once or twice.
Here's my thoughts:
PD3: Very easy
PD6: No enemies
PD11: Not too difficult, but the last level's a pain
PD15: Hard, avoid
PD26: A little harder than PD11
PD43: Medium difficulty,

at end
PD51: Quite hard, but

and

(gives and extra life) at end
PD72: Medium difficulty, last level's quite hard
PD72.2: Very easy
PD92: No enemies, and puts you at a better spot for 93
PD97: Hard, but it skips 98, which is harder

Also has
