The most stable element is iron, in terms of having the optimal binding energy (you gain energy for fusing lighter elements into iron, you gain energy for splitting heavier elements into iron). If proton decay doesn't exist, then the trend of all matter would be to turn into iron. If proton decay does exist, then iron itself would decay to less stable elements because the protons in iron would disappear, and the neutron-rich lighter elements would be subject to radioactive decay.

It is unknown how stable our stable nuclei are. Bismuth-209 was long listed as a stable nuclide, until a decay event was observed in 2003, establishing its half life of about 10¹⁹ years (by contrast, the universe is about 10¹⁰ years old).