If we don't wait for too long, where we don't know for sure, whether protons or atomic nuclei stay stable forever, there won't be just one single element.
The most abundant element in the universe will probably stay hydrogen, since it will stay in the intergalactic medium, and thin out by accelerating cosmic expansion.
The second-most abundant element will probably stay helium, mostly for the same reason.
The most frequent stars are probably red dwarfs.
They'll burn hydrogen and helium to oxygen and carbon, and end up as white dwarfs.
Only a small fraction of stars is large enough to burn oxygen and carbon to silicon and iron.
Very heavy elements, like uranium and beyond, as formed in supernovae, will decay to lead, helium (alpha particles), some hydrogen (via neutrons decaying to hydrogen), and a couple of less abundant elements formed by spontaneous fission. Other stable (isotopes of) elements heavier than iron are also formed in, or a consequence of, supernova explosions.
Part of the dead stars will collide, and merge to heavier stars, which will end as intergalactic dust (mixture of elements, e.g. iron and silicon), including planets, after supernova explosions, and as Black Holes. Others will be scattered out of reach of the supermassive black hole in the center of the galaxy (gravitational relaxation).