If we are talking exclusively about those basic elements, like in the 4-5-6 elements. Life fire, water and whatnot
Earth - It is universaly present and to me the most useful outside of combat, treating soil, building, even cleaning... Depending on how much it extends you can so some pretty amazing (and dowright scary) things.
If we are extending a bit and adding stuff like metal, poison... and basically going like pokemon.
Nature/life - I love the idea of creating and sustaining a garden, I just never had a good hand for it. Nature sounds like one of the most useful things as it makes things tastier and bigger, so yeah... love it.
If we are extending it to the absolute limit and saying any one word can be an element, like time, space, dimension.
my best pick for that is...
Luck - By far one of the most broken 'elements', like sure, you manipulate atoms and can cause a nuclear explosion... But I control the probability of you succeeding.
Just to clarify, I do not mean karma, I mean luck... the chance of things suceeding, not the opposite reactions the universe 'has to make' to balance it out.
It is an ultimate, 'i win' card at it's most powerful, literaly shaping reality to bend to your will without ever causing things like paradoxes or worrying about the possibility of failure, and even on it's lowest form, it would be heaven for anxious guy like me if I had a certainty that I had a better chance to succeed at just about anything.
Why yes... I did give this response and unatural amount of thought... How could you tell?