Which element has the atomic number 1?

Atomic number is simply the count of protons in the nucleus, and that count is what defines an element, so hydrogen, with a single proton, is number one and sits at the head of the periodic table. In its ordinary form the atom is about as simple as matter gets: one proton, one electron, no neutron at all. That simplicity matters historically, because hydrogen was the first atom whose spectrum could be explained theoretically, and the pattern of its spectral lines drove the development of quantum mechanics. It is also overwhelmingly the most abundant element in the universe, formed in the first minutes after the Big Bang and still the main fuel of stars, which fuse it into helium and release the energy that makes them shine. On Earth it is scarce as a free gas because it is light enough to escape the atmosphere, and is found instead locked into water and hydrocarbons.

Deuterium and tritium are hydrogen too; adding neutrons changes the isotope but not the element.