I like realism in war movies above everything else. But a well written and filmed story can make a war movie very watchable too. What I don't like are overly patriotic, unrealistic, stereotypical or badly researched war movies. Mel Gibson's "The Patriot" for example was partly insufferable.
I gotta go with "Band of Brothers" (though technically it isn't a movie). I really liked that mini series and am looking forward to "The Pacific", which is partly the same production team.
I also liked "Iô jima kara no tegami" ("Letters From Iwo Jima"), "Flags Of Our Fathers", "Saving Private Ryan", "Downfall - Der Untergang" and "Black Hawk Down". "Duell - Enemy At The Gates" and "Apocalypse Now" are pretty good too, in my opinion, though not always very realistic.
The battle sequences in "Braveheart" are also very good, albeit the rest of the movie is a romantic exaggeration mostly. Still a pretty epic picture.
I got to say something about "Downfall". I saw this movie in German and the acting of Bruno Ganz (playing Hitler) was downright frightening. To see somebody portray Hitler like that was disturbing. One moment he's a depressed, sad and physically sick man and the next a raging, delusional, raving lunatic. The way he spits while screaming and furiously gesticulates and a minute later stares at the wall and corrects his hair with this typical gesture left me speechless. And not just me. I saw this movie for the first time in a big movie theatre here in town and it was packed, but when the lights went on during the end titles the whole crowd was still stunned. It almost seemed like noone even dared to cough.