Okay, I shall ask this. Where will you find pure Oxygen? Pure Iron? Pure Sodium?
Correct me wrong, but outside of solutions, in which they are immersed in a solvent and the status of it will remain to be argued later, finding an element in a pure state naturally, since I'm assuming everyone isn't lugging around pure samples of their element, is near impossible. I leave the room for error regarding the elements in which a pure, stable state can be found, a minuscule chance regarding everything else.
And even within the realm of possibility, there's not much you can do. In a state like that where you do create the convoluted compound to manipulate another element, it would not necessarily be stable and would not have the same qualities if those elements were not bonded. I am still following the assumption that you can not 'harvest' your element from samples to gather a pure state due to laws regarding the conservation of energy and the instability that would possible occur from doing so.
In any case, Oxygen is still my choice and still an optimal candidate for manipulation. Given that I have access to pure Oxygen, I guess I'll just up the acidity of people's saliva, blood, etc. and watch their own body destroy them like that. I'd be able to deteriorate almost everything else as well.
As for isotopes and ions, for me, it depends on if it were naturally occurring. And outside of a solution. But all in all, I think it would be limited to, outside of compounds, to its most stable state as a pure element.