Ouch, that's tough because that is one of my favourite genres. Being British I feel obliged to say Dunkirk, and indeed it's a good film, but I'm also somewhat of a traitorous Brit in that I also enjoyed films like Patton, Band of Brothers, The Patriot, The Great Raid, Flyboys, Saving Private Ryan, Schindlers' List, The Boy in the Striped Pajamas, The Book Thief, Hotel Rwanda (those three are not war movies, but set in war times), Letters from Iwo Jima, Enemy at the Gates, Hacksaw Ridge, and so on.
So, a favourite...umm...Bridge Over the River Kwai...or really any war film. I just love watching them.