My favorite war movie? Well, that would be a tie between "The Longest Day" and "A Bridge Too Far"
First of all, Cornelious Ryan is a genious. He wrote the books those movies are based on.
To be honest, these movies are incredibly faithful to their books which in turn are incredibly faithful to the actual events that happened.
Downfall was also good in this regard. I guess I'm a bit of a nut when it comes to historical accuracty (Braveheart, did someone actually say Braveheart?).
For emotional impact, Platoon and Full Metal Jacket both are very good. Blackhawk Down was pretty good as well, and pretty faithful to the actual events.
Crap, I forgot Glory. That's a fantastic movie though I'm not really sure how accurate it was. Still, that movie kicks my ass every time I watch it.
Of course there's movies like the Pianist which is only kind of a war movie. Very good, but incredibly depressing.