Holy shit, you want me to pick just one line from the best parody movie of all time?!

I guess if I were forced to pick, it would be:
"I am your father’s brother’s nephew’s cousin’s former roommate."
"So what does that make us?"
"Absolutely nothing."

I'm surrounded by assholes!

Honorable mention because I use this one surprisingly often:

1-2-3-4-5? That's amazing. I've got the same combination on my luggage.

I agree with @ringwraithfish about that being a very hard choice.

...

Nope, can't decide between (all paraphrased because I mainly remember the German translation)

• "Comb through the desert!" [cut, followed by troopers using a giant comb on the sand]
• The passcode scene
• "Water, water!" "Water, water!" "Oil, oil!" "Roomservice, roomservice!"
• "Didn't I say only pack what we need for survival?" "But I can't live without this thing!"
• "Yoghurt! I hate Yoghurt!" [followed by underling offering to change Big Helmet's breakfast and a rebuke that that wasn't what he meant]
• "He's an Asshole, sir." "I know that, I want to know his surname!" "That is his surname, sir." [Big Helmet asks who else on the ship is an Asshole and nearly everyone holds up their hand.] "I knew I was surrounded by assholes!"

Comb the desert. "We ain't found shit!"

No more beaming! This time, I’ll walk! opens the door and it’s right there

"I'm not a human, I'm a mog!

Half human, half dog.

I'm my own best friend."

This will seem strange, because it isn't one of the quotable lines you usually think of.

But, "That's gonna leave a mark" when Barf forgets the seatbelt. That bit is so classically John Candy. It may not seem like it, but watch the scene and see how he plays it. Subtle, which is so rare with a bit physical humor.

I laugh as hard at the way he delivers that otherwise not very funny line as I do almost anything else in the movie.

It doesn't hurt that the line itself has become a family in-joke of sorts. We've got kids in the family that use the line when they bump into something or whatever, that haven't seen the movie yet.

Now, the scene that makes me laugh the most is the "oh no, not again" bit in the diner. The whole scene is great, what with that reference to alien, the dancing baby xenomorph, and the Pullman/Candy reaction.

But the seatbelt line encapsulates what I've always loved about Candy.

Such a great movie. To add to the list...

It's Mega Maid, sir. She's gone from suck to blow.

Light speed is too slow. We'll have to go right to Ludacris speed.
Ludacris speed?!?
What's the matter Colonel Sanders? Chicken?

We have gone plaid.

The Space Balls FLAMETHROWER!!!

When will then become now!?

Raspberry. There's only one man who would dare give me the raspberry! Lone Starr!

gestures broadly at entire script

Love the combing the desert scene.

https://www.youtube.com/watch?v=hD5eqBDPMDg