What is the molecular formula of sulfur?

The formula of the elements is best left to the simplest possible form, except in the case of the gases and the halogens, that are shown as diatomic molecules by tradition. The remaining elements are simply expressed as a single atoms, although it must be emphasised that elements, or rather the atoms of elements, do not ‘go around’ as single entities, unless they are noble gases.

All other atoms are unstable on their own under normal conditions. Metal atoms exist in giant structures, giant covalent elements exist as just that, giant and covalent, simple covalent molecules are arrangements of atoms in molecules etc. etc. Nothing exists on its own.

So, if we want to represent carbon we don’t write down C_{(very big number)} we just understand that it is a single macromolecule and express it as C, pure and simple.
The same goes for all giant structures, be they metals, or macromolecules. In the case of phosphorus, sulfur and certain other simple molecular elements we are faced with a choice. Do we refer to them by the molecular formula, or do we understand that they exist as finite simple molecules, but continue to express them as if they were free entities? Most texts and chemists choose the latter.

It is understood (within the chemists fraternity) that sulfur is usually found as S₈ molecules and that phosphorus is P₄ pyramids, but that there is no real advantage to be had expressing them as such. Indeed sulfur changes its form several times while being heated through to the vapour state.

S₈ crowns give way to S₈ strings that entangle into S₈ cross linked structures that turn to S₈ vapour.

When sulfur is reacting with (for example) oxygen which is the ‘correct’ way to represent it? What form does it actually have at the moment of reaction? We don’t know (and probably don’t care).
The sensible answer to the first question is that there is no ‘right’ way to express the chemical structure. For this reason the majority of texts (and chemists) simply refer to sulfur as ‘S’, using the same logic as used for giant and similar structures.