Based on that criteria, maybe this is the greatest line.
A Simple Plan wasn’t even all that great a movie, but man this line killed me. Bill Paxton (Hank), Billy Bob Thornton and Billy Bob’s buddy found a crashed plane, dead pilot and a bunch of money deep in the woods. A cop is approaching down the road. The three men agree not to say a word about the plane. Billy Bob waits in the background as Hank smalltalks with the cop. Then Billy Bob slowly approaches and says…
“Hey Hank, did you tell him about the plane?”
“As God is my witness, I thought turkeys could fly.”
WKRP in Cincinatti
Thank you!!! I wanted to post this one, but had already posted one. I’ve since come to realize movie quotes are like potato chips, sorta. You can’t post just one.
You gotta space them out a little; like, only one a day. 
In There’s Something about Mary, Ben Stiller asks Chris Elliott , “So, how’s married life?”
Chris’s deadpan response… “Each day’s better than the next”!
From the Watchman, Rorschach in jail, just after he dunked someones head in hot oil
" None of you seem to understand. I’m not locked in here with you. You’re locked in here with ME!"
Ah. One of my favorites. Since that’s been used, I’ll sumbit:
Alright you Primitive Screwheads, listen up! You see this? This… is my boomstick! The twelve-gauge double-barreled Remington. S-Mart’s top of the line. You can find this in the sporting goods department. That’s right, this sweet baby was made in Grand Rapids, Michigan. Retails for about a hundred and nine, ninety five. It’s got a walnut stock, cobalt blue steel, and a hair trigger. That’s right. Shop smart. Shop S-Mart. You got that?
-Ash
Too bad that game lines aren’t included :(, but if they were, I’d say hands down the best line ever to grace my ears would be:
“Men, we led those dumb bugs out to the middle of nowhere to keep ‘em from gettin’ their filthy claws on Earth. But, we stumbled onto somethin’ they’re so hot for, that they’re scramblin’ over each other to get it. Well, I don’t care if it’s God’s own anti-son-of-a-bitch machine, or a giant hula hoop, we’re not gonna let 'em have it! What we will let 'em have is a belly full of lead, and a pool of their own blood to drown in! Am I right, Marines?” -Johnson’s speech in Halo on Legendary
If it has to be movies or TV:
“I know what you’re thinking. ‘Did he fire six shots or only five?’ Well, to tell you the truth, in all this excitement I kind of lost track myself. But being as this is a .44 Magnum, the most powerful handgun in the world, and would blow your head clean off, you’ve got to ask yourself one question: Do I feel lucky? Well, do ya, punk?” -Harry Callahan, Dirty Harry
I was SO hoping for a name change when Sears and Kmart merged.
Two from scenes with Danny DeVito:
Jack (Kevin Spacey): How’s it hangin’, Sid?
Sid (DeVito): Down around my ankles.
(L.A. Confidential)
and
Mickey (DeVito): Don’t you wanna hear my last words?
Joe (Gene Hackman): I just did.
(Heist)
Surprised I’m the first to post it this, hopefully it’s not too long:
+1
I also like “it’s too bad she won’t live. But then again who does…”
Several I could heartily agree with have been posted upthread, so i will just add:
“Who ARE those guys?”
-Butch Cassidy and the Sundance Kid
Well, I haven’t read all of these, but I bet (one of) my favorites hasn’t been quoted:
The fourth Doctor talking to Leela:
“The very powerful and the very stupid have one thing in common. They don’t alter the views to fit the facts. They alter the facts to fit the views.”
I think you meant to say
“Think you used enough dynamite there, Butch?” instead. 
That was a good one.
“I’m gonna need a hacksaw.”
Jack Bauer, 24
Dr. Necessiter: As you know, my research has advanced to a point where I can put her mind into the body of a gorilla.
Dr. Michael Hfuhruhurr: I couldn’t fuck a gorilla.
The Man with Two Brains
“Love is too weak a word for what I feel - I luuurve you, you know, I loave you, I luff you, two F’s, yes I have to invent, of course I - I do, don’t you think I do?”
Alvy Singer from Annie Hall.