In this world of ours (i.e. to the limits of the known universe), by its very nature a positron has no real 'chemistry' besides going BOOM when coming in contact with an electron.
A positron is not just a 'positively charged' electron...it IS anti-matter.
When we ask what is the most important sub-atomic particle...you must first ask yourself, what does 'important' mean?
Important to me mean, that which determines the majority of physical and chemical properties of a substance...by this definition, the electron is by a long stretch the most relevent, both in the classical realm and the quantum realm.
Think about all you understand about chemistry and ask yourself, how many of the concepts are a consequence of the nature and behaviour of electrons...