I'm going to use directions here in relation to the letter's perspective, so something described as "above" a U means it is on the side adjacent to the "top" of an upright letter U.
Additionally, I'll use numbers to describe the dice (left to right) so P1 refers to the P on the first die, while L5 would refer to the L on the fifth die.
First pair of consistent dice:
The first and fourth are consistent, as they are basically a rotated view of the same three sides. Note that in the first the side that looks like an N ends up looking like a Z in the fourth, due to the orientation. In both of these, the U side has a P "below" it and the Z/N to the "left" of the U. So we can ignore the fourth, for now.
Second pair of consistent dice:
The third and fifth dice are similarly consistent, as there is an L to the "left" of the U and a Z/N "above" the U. The orientations of all letters are correct, so we'll ignore the fifth die.
What about those two pairs compared to each other?
But notice that the U3 cannot be the same as U1, as U1 has an N/Z to the left and U3 has an L to its left. So either these two pairs are inconsistent, or they are showing two different letter U's on the die.
One of the remaining two:
Now, U2 has an L "above" it. This means U2 can't correspond with U3, but it can correspond with U1. This would put L2 opposite P1 and an E2 opposite N/Z1. For die 3 to be consistent, U3 can be opposite U1, and have N1 "above" it.
And the last one?
This puts a U to the "left" of P1, which is inconsistent with P6, so P6 is the only one that doesn't fit with the rest.
Why this one is tricky (in my opinion):
The trick to this puzzle is that you end up assuming the letters are P-U-N-Z-L-E when in reality they are P-U-U-Z/N-L-E.