I'm currently watching Full Metal Jacket. I consider it to be 2 movies though, 45 minutes at boot camp, and then the rest in-theater.

Not necessarily my favorite, but the first 45 minutes is probably some of the best movie footage of all time.

EDIT:
Holy shit you guys named at least 103 movies. I took the ones with the most mentions and put them in for a vote via a poll.

Here is a list:
12 O'Clock High
13 Hours
'71
A bridge too far
All's Quiet on the Western Front
Apocalypse Now
Army of Darkness
Band of Brothers
Battle of Britain
Beau Geste
Black hawk down
braveheart
Breaker Morant
Bridge Over The River Kwai
Casualties of War.
Catch 22
Clint Eastwood's two films on Iwo Jima
Courage Under FIre
Crimson Tide
Das Boot
Dirty Dozen
Down Periscope
Downfall
Dr. Strangelove
Enemy at the Gates
Fighting Seabees
Forrest Gump
Full Metal Jacket
Fury
Gallipoli
Gettysburg
Gods and Generals.
Good Morning, Vietnam
Green Berets
Gunga Din
Heartbreak Ridge
Henry V. Epic war movie and Shakespeare play
Hogans heroes
Humphrey Bogart's Sahara
In Harm's Way
Inglorious Bastards
Kagemusha
Kelly's Heroes
Kilo Two Bravo
Last of the Mohicans
Lawrence of Arabia
Longest Day
M*A*S*H
Mad Max
Master and Commander
Mega Force
Memphis Belle
Midway
Ministry of Fear
Pacific.
Paths of Glory (Kirk Douglas at his best)
Patriot
Patton
Platoon
Predator
Red Dawn
Run Silent Run Deep
Sands of Iwo Jima
Saving Private Ryan
Schindlers List
Sergeant York
So Proudly We Hail!
Spies like us
Stalag 17
Tae Guk Gi
The Beast
The Bridge Over The River Kwai
The Charge of the Light Brigade
The Dam Busters
The Dirty Dozen
The Dogs of War
The Good the Bad and the Ugly
The Great Escape
The Green Berets
The Guns of Navarrone
The Hunt for Red October
The Killing Fields
The Kingdom
The Longest Day
The Pacific
The Patriot
The Thin Red Line
The Wild Geese
They Marched Into Sunlight
They Were Expendable
Thirty Seconds Over Tokyo
Three Kings
To Hell and Back
Top Gun
Tora! Tora! Tora!
Troy
U-571
War of the Roses
When Trumpets Fade
Where Eagles Dare
White Tiger
World at War narrated by Laurence Olivier
Zulu

Humphrey Bogart's Sahara.

Wow picking one is an incredibly tough choice:
The Thin Red Line

Runners-up:
All's Quiet on the Western Front
12 O'Clock High
Bridge on the River Kwai
Lawrence of Arabia
Das Boot
The Longest Day
Saving Private Ryan
Memphis Belle

Usually watch Full Metal Jacket from the haircut scent until they play "These Boots Were Made for Walking: .
Didn't really care for the rest.

"Band of Brothers" is great.

Right now, I'd say Fury

"To Hell and Back" Audie Murphy had the lead role about himself and his actions during WWII.

FMJ
Top Gun is pretty good too, although not sure it's considered a war movie.

Agree sir!
1 - Run Silent Run Deep https://www.youtube.com/watch?v=TKM7CzLytxc


Runner-up: Bridge on the River Kwai https://www.youtube.com/watch?v=4k4NEAIk3PU

Both take me back to watching them as a little boy with my father.


Favorite war documentary - World at War narrated by Laurence Olivier
1st Episode- https://www.youtube.com/watch?v=Q3H0K0Afk20

World at War is a fantastic documentary series. I remember watching most of the episodes as a kid in Elementary School and discussing it with my Dad at the time. One of the things I remember most were the interviews with veterans and survivors on all sides of the conflict.

Can't pick one really. There are some great ones, middle tier, then all the rest.

All have minor errors if you want to nitpick, but thinking big picture:
In the "great ones" category I'd say:
Band of Brother series. 10 mini-movies if you will. Outstanding on all accounts and for the variety and character development.
Saving Private Ryan. Strong action and acting performances, excellent character development.
Platoon. I enjoyed the narration, the chaos, the senselessness of Vietnam, etc.
Full Metal Jacket (1st half is fantastic, 2nd half pretty dull and borderline stupid at times).
Black Hawk Downs is a solid film, beginning to end.

Middle Tier - A few that were alright, but not great:
The Pacific was disappointing, with some good sections.
The Lost Battalion is an okay WWI movie staring Ricky Schorder. It's slow but I enjoyed it. More about a person and his struggles with the war.
We Were Soldiers. I wanted to like it and it had some good moments, but some really hokey moments too.
Hamburger Hill - I wanted to like it since I served with the 101st, but it was just really dumb in spots. A lot of bad over-acting.
Enemy at the Gates gets a nod as I recall it being a good film.
American Sniper was decent, albeit it seemed like a propaganda film for 1 individual.


Waiting to see:
I've yet to see "Lone Survivor" but read the book. Waiting for it on free streaming.
I've yet to see Fury but it looks good.

The Beast and Fury are two of my favorites.

Edited to add Saving Private Ryan.

If you consider a series to be a movie then Band of Brothers, pretty much hands down.

Memphis Belle is an old favorite as I really enjoy planes and the utter chaos of the aerial campaign.
Fury is a current favorite as I've gotten really into World of Tanks Blitz gaming lately.
Saving Private Ryan is another favorite showing what the troops on the ground went through.

Good Morning, Vietnam because fuck it why not.

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Enemy at the gate
Memphis Bell
Green Beret.
And a bunch of others.

Band of Brothers. Excellent story and excellent character development. The book is amazing too.

The Pacific was good too but not as good a BoB in my opinion.

I have yet to see Unbroken so that may be a runner up. But I can say the book was very good. I cant believe what those guys went through....and survived.