What is the most common molecule in living things?
I'm going to say water.
The concentration of water in water is 55M, so even if something is like 20% water, at the level of moles, water is super abundant. Since we are ~70% water, it is the predominant molecule.
Almost all life depends on the information recorded in the DNA molecule. Some virus have RNA for information but it is debatable if viruses are actually living things. Also there is a theory that RNA came from DNA by a degenerative process of evolution.