Cross of Iron, Stalingrad (German film), Downfall (German film), Aces High, Thin Red Line (controversial choice), Das Boot (suppose a series not film), Waterloo, Zulu, We were soldiers once and young, Apocalypse Now (cliche yawn choice, but amazing), Regeneration, Birdsong(two part series), BBC Our War series esp first episode set at Mons (easy best WW1 dramatization out there) are my favourite films, the recent film about the pacifist medal winner was cool.
some decent Russian films recently like the Brest Fortress one, but expect propaganda with Germans being evil useless cannon fodder. Decent film about Warsaw Uprising recently too.
When Trumpets Fade is good for a lower budget film.
not about fighting but WW2 you can't ignore Shindliers List