Sharona Fleming in the pilot for Monk, in reference to politicians:
I never vote. It just encourages them.
You’ve got me going down a Shakespeare rabbit hole. There are so many countless memorable lines from the Bard many of which we still use today. I enjoyed this one I hadn’t read before:
“I can see that he’s not in your good books,” said the messenger. “No, and if he were I would burn my library.”
I’ve been watching all of the Seinfeld episodes back-to-back, and there are so many great lines. While walking the dog the other day I had a thought similar to the OP’ question, but what’s the best single line that’s only two words long. I’d go with “I’m out!”
“If you trust in yourself. . .and believe in your dreams. . .and follow your star. . . you'll still get beaten by people who spent their time working hard and learning things and weren't so lazy.”
Terry Pratchett
“The enemy," retorted Yossarian with weighted precision, "is anybody who’s going to get you killed, no matter which side he’s on, and that includes Colonel Cathcart. And don’t you forget that, because the longer you remember it, the longer you might live.”
“You use a glass mirror to see your face; you use works of art to see your soul.”
― George Bernard Shaw, [Back to Methuselah]
“It is possible to commit no mistakes and still lose. That is not a sign of weakness…that is life.” ----Jean Luc Picard
Also from Picard:
“I think when one has been angry for a very long time one gets used to it and it becomes comfortable like old leather. And finally it becomes so familiar one can’t ever remember feeling any other way.”
“Make one mistake and you pay for it the rest of your life.”
—Snoopy
My most recent favorite line.
A couple of days ago I watched, on Netflix, Gangubai Kathiawadi . It is a thoroughly enjoyable Hindi biography/crime story. Gangubai is on her way to give a speech and her assistant tells her to seek God’s blessing.
Dear God. You’ve let me down so many times, don’t let me down today.
‘Instant gratification takes too long.’
–Postcards from the Edge.
‘I’m pretty much fucked.’
–The Martian
‘You know how to whistle, don’t you, Steve? You just put your lips together and blow.’
–To Have and Have Not
“If you mention extortion again, I’ll have your legs broken.” - The mayor in Animal House
“Gilbert! I think sexual intercourse is in order!”
(The social climbing wife, played by Maggie Smith, in A Private Function)
More romantically:
“Thank you for coming back to me”
(Final line of Brief Encounter)
Not forgetting
“Well, nobody’s perfect”
(Some Like It Hot)
“I always hear ‘Punch me in the face’ when you talk, but it’s usually subtext.” – John Watson, Sherlock
A great line to randomly toss out. People either know it or they don’t. You learn something about them by their response.
It was on my mind because we just saw this in the Dali museum in Paris a week or so ago:
A clear homage to Magritte, and for extra fun Dali wrote “this is not a plate” on an actual plate. 
“I don’t trust anything that bleeds for five days and doesn’t die”
Mr Garrison, South Park: Bigger, Longer and Uncut
“It is a sad and beautiful world!”
Robert Benigni’s character Roberto in Down By Law
“I pulled into Nazareth, was feeling 'bout half past dead”
(The Weight)
“She tried to sit in my lap while I was standing up.”
(The Big Sleep)
As a closing line, I prefer
“Stand still, Godfrey. It’ll all be over in a minute.”
(My Man Godffrey)
From Dr. Strangelove:
“Gentlemen, you can’t fight in here, this is the War Room!”
A close second:
“You’re going to have to answer to the Coca Cola Company.”