Although R Lee Ermey is the epitome of the drill instructor, I think Eve hit the nail on the head with Lon Chaney as the first. The U.S. Marines thought he was so convincing that they made him an honorary Marine. There was even a Marine honor guard present at his funeral (he was just showcased on PBS’s American Masters a week or two ago).
I’ve seen a lot of 'em but R. Lee in Full Metal Jacket was truly scary. So was Donofrio.
I agree with Poysyn about Louis Gossett Jr. in An Officer And A Gentleman.
/Foley/
“*I don’t believe what I’m seein’. Where’ve you been all your lives, *at an orgy?!!? Listening to Rolling Stones music and bad mouthin’ your country I’ll bet.”
<hijack> It has been 27 years since I was in Parris Island, Senior Drill Instructor Staff Sergent Kenneith L. Saunders voice still rings in my ears. <end hijack>
Ermey again in the pilot episode of “Space: Above and Beyond”
“Everybody reach to your right and grab the ass cheek of the person next to you. Got a good, tight grip? Good. Now listen. That ass you’re holding onto is your ass. It belongs to you. Your duty is to get that ass of yours back in one piece. Understand?”
Was it?
I liked James Whitmore in *Battle Cry[/].
I’d personally be most afraid of Sargeant Detritus.
From “The Siege of Firebase Gloria:”
“I have no respect for these men.”
But you have to see it to put it in context.
**Firebat023[b/] drop and give me 30 for making me laugh out loud at work. Man you brought back some funny memories. Well they’re funny now…at the time, not so much.
I graduated Army basic and advanced (OSUT) June 1988 2nd platoon, Alpha Battery 1/19 Field Artillery. I saw Full Metal Jacket the day before I left for basic. I was surprised at how similar my basic training experiences were to the movie. Drill Sergeants must attend a creative insult class. Something like a Don Rickles school of obscene insults.
Yeah…watching FMJ and then going to training is a bit like the old pie in the face routine: It stops being funny when it starts being you. 
A slight hijack:
I was a civilian cook at OCS at Quantico during the summer of 1985 and had to watch as the instructors ground the candidates into the dust. My favorite was a monster named, and I’m not making this up, Ssgt. Harold Melvin (I always wanted to ask him “What happened to the Blue Notes?”). SSgt. Melvin, in a snit, tried to berate me like one of the candidates to which I replied “I’m not one of your candidates you fucking ape, leave me the hell alone or I’ll piss in your SOS.” He laughed and we got along pretty well after that.
R. Lee Ermey:
“Drop your cocks and get your socks.”
This one from Full Metal Jacket cracks me up.
"I bet you’re the kind of guy that would fuck a person in the ass and not even have the god damned common courtesy to give him a reach around. "
Philly
Ermey has another terrific line in that episode when asking a female recruit why she joned the service:
Recruit: “To fight our enemies, sir!”
Ermey: “Enemies? What enemies? We aren’t at war, we don’t have any enemies. Are you insane?”
The “Space: Above and Beyond” episode was neat because he did the D.I. schtick without actually swearing, beyond “ass.” And yet it was still scary and funny as hell.
Of course, anyone who saw him in “Dead Man Walking” knows the man really can act.