Overleaf
Overleaf is a popular choice and it is quite elegant in the sense that in Overleaf, they have used LaTeX as their backend an and lets let ou u you ue use severa packages that yu can use to structure your docuent. The upside is tha that, ther eare there ar are numerous temlates, LaTeX itsself if is very powerful. the downside is that LaTeX is still quie quite steep to learn.
Google Docs, Online Word, Others