Re: Canon de 155 L Modèle 1877 and Canon de 120mm Mle1878
Would also add to these excellent suggestions the canon de 105mm Schneider Mle 1913 which served not only in WW1 but was also the principal gun of the French Army in 1940. It was exported widely between the wars to Poland, Yugoslavia and Czechoslovakia and Belgium.
Plus, another vote for artillery crews.
The old French artillery pieces suggested in this thread would be my third choice. Useful, given how long they were used, and produced in substantial numbers. Regards, Pa