Wasn't France considered to have the best modern army on the planet by 1918?

No, they weren't I'm afraid. That was the BEF, which took the most amount of casualties from 1916 onwards, way out of their proportion especially at the somme, and absorbed the spring offensive and spearheaded the Hundred Days. Only sized 1.9m they took almost the same number of prisoners and guns as not only the French and Belgians combined, but also the Americans too (4.7m), whilst defeating the Ottoman Empire in Israel on top of that, without a single front line mutiny

https://en.wikipedia.org/wiki/British_Army_during_World_War_I#Western_Front