There are lot of movies have watched till a day but some of mine top rated & favorite movies are:

Where Eagles Dare
The Guns of Navarone
Dirty Dozen
Great Escape
Full Metal Jacket

I do not like the typical war movie bro story, so I'll just go with this

'Allo 'Allo
Der Untergang
Victory

Battlestar Galactica (2004 TV series)

Apocalypse Now
Das Boot
Kelly's Heroes
Merry Christmas, Mr. Lawrence
MASH
Galipoli
Enemy at the Gates

Shucks, I forgot about Enemy at the Gates!

Just to add a couple of movies not mentioned yet

The Bridge on the River Kwai

Paths of Glory

Roma, città aperta

Cross of Iron

 Btw, for those that say M.A.S.H., do you mean the movie, the TV Series or both?

300

Movie for me. Altman at one of his best.

1. Blackhawk Down
2. Letters From Iwo Jima

Of the two, I prefer BHD. I have a few prints of Shughart & Gordon hanging on the walls of my apartment, they are heroes of mine.

Do documentary series count? Cause this is a must see:

Other than that, I think my favorite war movie is Patton.