K-40!! Naturally occurring radioactive ⁴⁰ K decays with a half-life of 1.248×10⁹ years. 89% of those decays are to stable ⁴⁰ Ca by beta decay, whilst 11% are to ⁴⁰ Ar by either electron capture or positron emission.
Obligatory XKCD File: https://commons.wikimedia.org/wiki/File:Radiation_Dose_Chart_by_Xkcd.png Which shows the Banana Equivalent Dose.