What are some of your favorite lines in scifi history (movies, tv, books, …)?
I will have to go with Han Solo’s answer to Leia’s “I love you!” while being plunged into carbonite at the end of “Empire Strikes Back”:
Leia: “I love you!”
Han Solo: “I know.”
This might also win the best romantic line in movie history 
Close runner-up:
Ellen Ripley (at the end of “Aliens”, talking to the Alien Queen who’s threatening a little girl): “Get away from her, you bitch.”
Interesting fact about the “I love you/I know” exchange: it was improvised. The script had Han Solo saying something like, “I love you too,” which Irvin Kershner (the director) and Harrison Ford found untrue to the character. After a few takes experimenting with various replies, Ford came up with the “I know” retort.
A favorite line of mine, also from Star Wars, is C-3PO’s reply to R2-D2 in The Empire Strikes Back when R2 is trying to open the door by sticking his little computer link thingy into what turns out to be a power socket: “Well don’t blame ME. I’m an interpreter. I’m not supposed to know the difference between a power socket and a computer terminal!”
That’s a toughy…
A couple of my favorites… none can claim all-time status I don’t think, but I have quoted and re-quoted them… 
“You know what the chain of command is? It’s the chain I go get and beat you with until you understand who’s in ruttin’ command here.” - (Jayne from Firefly)
“Algorithms I trust. Boolean logic I trust. Beautiful women… they just mystify me.” (Zane from The Arrival )
“Gentlemen, you can’t fight in here! This is the War Room!” - Dr. Strangelove
“Khaaaaaaaaaaaaaaan!!!”
“Will someone get this walking carpet out of my way?” - Princess Leia
“Laugh it up fuzzball” - Han to Chewie
More Star Wars:
“Leia: I’d just as soon kiss a wookie!”
“Han: I can arrange that. You could USE a good kiss!”
Futurama:
(from the episode “Roswell that Ends Well”)
Prof. Farnsworth: “Oh, advice on not messing with history from mister ‘I’m my own grampa.’ SCREW HISTORY! I wanna go home.”
Another Futurama one (from “The Series has Landed”):
Bender: “I’ll build my own theme park. With blackjack. And hookers. In fact, forget the theme park!”
Amen. Amen.
“You’re terminated… F**ker.”
– Sarah Conner The Terminator
Old fave: “Reverse the polarity of the neutron flow!!”
Dave: Open the pod bay doors, HAL.
HAL 9000: I’m sorry Dave, I’m afraid I can’t do that.
&
Terminator: Come with me if you want to live.
Helen: Gort! Klaatu barada nikto!
and one more, because I can’t help myself.
John Bigboote: It’s Big-booTAY, Big-booTAY!
Hard to pick an all time favorite, but a few come to mind:
Benjamin Sisko: Where’s Tolar?
Garak: I’ve locked him in his room. I’ve also given him the distinct impression that if he tries to force the door open, it may explode.
Benjamin Sisko: I hope that’s just an impression.
Garak: It’s best not to dwell on such minutiae.
-Deep Space Nine
The avalanche has already started. It is too late for the pebbles to vote.
-Ambasador Kosh, Babylon 5
K’tano: I see you are one who speaks your mind, O’Neill.
O’Neill: Yes, which is why I don’t say much.
-Stargate SG1
Q: For that one fraction of a second, you were open to options you had never considered. That is the exploration that awaits you… not mapping stars and studying nebula… but charting the unknown possibilities of existence.
Q: We’ve all been the scarecrow.
Tigh: Which side are we on? We’re on the side of the demons, chief. We’re evil men in the gardens of paradise, sent by the forces of death to spread devestation and destruction wherever we go.
Almost everything by Garak is just pure gold, this exchange between Garak and Bashir is one of my favourites.
Garak: “Why is it no-one ever believes me, even when I’m telling the truth?”
Julian: “Have you ever heard the story of the boy who cried wolf?”
Garak: “No!”
Julian: “It’s a children’s story. A young shepard gets loney while tending his flock. He cries ‘Wolf!’ and the townspeople come rushing to his aid. When they discover there is no wolf, he claims he scared the wolf off, and they congratulate him for his bravery.”
Garak: “What a clever boy!”
Julian: “There’s more. The boy did the same thing the next day, and the day after, and the day after. And on the next day, when a wolf really did come, the townspeople didn’t come. They’d gotten tired of his lying. The wolf ate all the sheep and the little boy.”
Garak: “Isn’t that a bit gruesome for a children’s story?”
Julian: “The moral is that if you lie all the time, people won’t believe you even if you’re telling the truth.”
Garak: “Are you sure that’s the moral?”
Julian: “Of course. What else could it be?”
Garak, leaving: “Never tell the same lie twice.”
Another exchange with Bashir
Bashir: So of the stories you told me, which ones were true?
Garak: My dear doctor, all of them were true.
Bashir: What about the lies?
Garak: Especially the lies.
Yet another one
Bashir: Assuming you’re not a spy.
Garak: Assuming.
Bashir: Then maybe you’re an outcast.
Garak: Or maybe I’m an outcast spy.
Bashir: How could you be both?
Garak: I never said I was either.
Now Odo and Garak
Odo: You’d shoot a man in the back?
Garak: It’s the safest way, isn’t it?
And one quote I particulary love (I try to use it whenever possible):
Garak: The truth is usually just an excuse for a lack of imagination.
How many Starfighters do we have left?
Including yourself?
Yes!
One.
I’ve always wanted to die fighting a desperate battle against impossible odds.
When someone asks if you’re a god, say yes!
Total Recall:
“Sorry sir…you’re breaking up…must be thunthpots…”
Total Recall:
Mutant to Arnold Schwarzenegger: “You have a lot of nerve showing your face around here…”
Arnie: “You can talk!”
Predator:
After Arnie impales a rebel with a knife to a tree: “Stick around”
On a serious note, Blade Runner:
Roy Batty’s final monologue.
“All these memories will be lost in time…like tears in rain. Time to die”.
Speaking of Total Recall quotes…
“See you at the party Victor!”
“Screeeeeew yooooooou!”
Those quotes are hilarious when envisioned either in context or out of context. 
A couple of my favorites:
Red Dwarf:
“Everybody’s dead Dave!”
Escape From New York:
“Snake Plissken. I’ve heard of you…I’ve heard you were dead!”
Remo Williams:
“In Korea, door handles do not break!”
Tripping the Rift:
“Your mother gave me a hummer!”
That last one is just funny in the context of the show, and I laugh every time I think about that one. Some of these might be considered boarderline Sci-fi shows, but they’re still great shows in themselves!
J
I was just about to add this one myself.