re: In search of a really good war movie

WWII
The Bridge on the River Kwai (1957)
The Great Escape (1963)
Von Ryan's Express (1965)
The Train (1965)
The Guns of Navarone (1961)
The Longest Day (1962)

WWI
The Blue Max (1966)

Not a movie but Band of Brothers (the European one) was one of the greatest war series ever.

Band Of Brothers or Patton

Memphis Belle
Enemy At the Gates
Good Morning Vietnam
Generation War
Rambo III
Apocalypto
Dances With Wolves
The Last Samurai
In The Army Now
U571
Edge of Tomorrow
Gettysburg
13 Hours
Zero Dark Thirty
Black Hawk Down
Schindlers List
Restrepo

Battleground (1949)

Vietnam:

Full Metal Jacket

The Boys of Company C

We Were Soldiers

Twelve O'Clock High. End of discussion.

But I will add a few more:

Best Years of Our Lives

Stalag 17

Bridge on the River Kwai.

Many of the best war movies ever made have little or no combat.

Where Eagles Dare

Run Silent, Run Deep

As a combat vet, my two favorites are Restrepo (for a real life documentary) and Fury (for a fictional version of the social and the psychological aspects). I thought The Pacific did an extraordinary job as well.

I think virtually all the rest of the modern stuff (13 Hours, Hurt Locker, etc.) is fricking awful.