pigletto #5: I say

because it is the only one without the letters o and f in its name

billy
4: If it wasn't so difficult, I would guess

because it is not needed for any quest while the others are...
5:

because the others are commonly regarded as very good items and are all (directly or indirectly) worth a lot of points
Enough for now... I have an idea for 2), but I think I should wait a bit
