That pure and unbound state stipulation means they would all be fucking useless. You'd only be able to really work with inert gases since almost everything else bonds with itself or other things rather than remaining pure and unbound. I dunno about you, but I think Helium/Neon/etc. bending would be pointless.
If it were something along the lines of only being able to manipulate that element, regardless of the purity of what it's in and whether or not it's bound with itself, I would choose Oxygen because I could then suffocate people by pulling all the O2 out of the air around them.
He said he just meant purely on element, and that the bonding is irrelevant.
Hydrogen is an element
H+ is a form of hydrogen that happens to be a single proton.
I now control all protons, and thus all elements.
I repeat, GG WP.
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He said he just meant pure, in which case Hydrogen is the only logical route.
I have to disagree in a sense, because we're already defying logic through controlling elements, so from there it wouldn't really be a stretch to say all elements are a compound of H+(protons).
Either way, the only logical choice is still hydrogen because then you can control stars.
But yeah, that's a good argument for it. Controlling stars would be pretty boss.
I'd pick any of the noble gases. Why? Because they are naturally stable and don't typically bond. Second: All those lights filled with pure noble gases. Ungh.
EDITING: Why I say lights is... Can you imagine all the exploding lightbulbs? Orrr.. ya, I'm not sure, but certain ones in enough concentration can be deadly.