This is kind of a silly question. Due to the way WaniKani works, our reviews come in “waves” that usually have similar SRS stages. I can see these “wave stages” in the Smouldering Durtles forecast (not sure if regular WaniKani shows it easily). And I’ve noticed that different stages have different confidence levels.
For example, if I have a wave of 10 items going from Master → Enlightened, I might be a bit slow on some of them, and I might get a few of them wrong.
I think that Guru 1 → Guru 2 and Guru 2 → Master might be even worse… maybe that is where the most backsliding happens.
Actually the absolute worst is Apprentice 1 → Apprentice 2, specifically for kanjis in the higher levels (where they represent increasingly abstract semantic concepts). At my current level (27) I’ll often fail that stage 3 times in a row over 24 for hours before finally achieving “short term burn” and being able to move forward.
On the other hand, I think either Apprentice 3 → Apprentice 4 or Apprentice 4 → Guru 1 are probably the easiest. When a wave of those comes, I blow through with super high speed and super high accuracy… because I’ve seen all of them many times in the past few days. These waves feel fun, and I look forward to letting each little durtle surf these waves… and I simultaneously get sad for the disappointment these durtles will feel when they hit Master and experience their first big wipeout. 
