Personal preference used to be Pure Shadow or Lost Souls both in PVP and PVE.
I think that there isnt really a strongest element in the game as they all have their pros and cons and in the end there is always a counter for everything. Its more personal preference in my opinion.
But if i had to pick an element that is the strongest for PVP i would probably say that its Fire.
The elements are pretty balanced as far as i can remember so its hard to say that there is such a thing as strongest element.