Well, the most famous artist active during WW1 would have been Pablo Picasso who was going through his "Synthetic Cubism" period at the time of the war.
If you mean the biggest artist who painted WW1 themes, I would suggest Sir Stanley Spencer (see link 1 and 2), although the most famous WW1 painting is probably "Gassed" by John Singer Sargent (see link 3)
