The most boring element is EQVE. It's just more boring DUST, but since it's indestructable it won't go in Q3.
The element I have the least fun playing with is PLNT.
The most useless element is PLNT. It can pretty much only be used to heal STKM, which is a pretty boring use.
EQVE
PLNT
PLNT
i think you meant PLNT not PLANT