1917 and Private Ryan are both amazing. The first really shows what was it like, I remember at times thinking "mh, I don't really remember that much about WWI", well, there was not that much to remember, it was mostly trench warfare with people dying uselessly for this or that scrape of no man's land. WWII has more definite and famous moments - the nazi, the allies, operation Barbarossa, the D-Day, Dunkirk, etc... but WWI was basically people spending 4 years shooting each other from a trench to the other until someone somewhere signed an armistice.