Buffy, about Dawn, in All The Way. “Dawn’s in trouble? It must be Tuesday.” 
“Are you keeping up or do you need some kind of English-to-Constant-Pain-in-My-Ass translation?”
Willow? Yes.
But you’re off by a number of years. Willow was much younger when she said this quote.
This one has not been answered yet, and is still in play.
It’s Willow, in Becoming part 1. Before the first time they attempt the re-ensoulment of Angelus.
She’s talking to Buffy.
I’ll leave silenus’ quote in play.
Xander to Spike and Buffy in “Him”, the one with the ultimate letter jacket.
“Can’t this thing go any faster? Ultimate driving machine, my ass.”
To nitpick: It’s actually in Once More, With Feeling after Buffy finds out Sweet is holding Dawn hostage at the Bronze.
Buffy, to Riley, as he floors it trying to get to CoolXander and GeekXander, who are both with Anya, in “The Replacement.”
Oh, and my next:
**I’m going to give you all a nice, fun, normal evening if I have to kill every single person on the face of the earth to do it. **
Buffy to the rest of the gang in The Prom.
I’ll add a new quote in a few minutes, once I think of something.
I like your coat. When does the Slayer find time to shop?
That would be Amy the Witch, de-ratted, to Buffy in Wrecked.
“You’ll be doing limbo in no time.”
Xander to (paralyzed by the Gnarl) Dawn in Same Time, Same Place
“You’re gonna think about that later, mister, and you’re gonna laugh.”
Buffy to the gang (especially Giles) in “Go Fish”, about her “demons with high cholesterol” crack.
“It is not Bob Barker, scabby morons!” (easy, I know, but funny)
Glory to her minions, especially Jinx, in Intervention.
“Oh, well, right now I’m imagining you in jail. You’re wearing a big orange suit, and, oh look, the guards are beating you up.”
Buffy to the swim coach in Go Fish. I think?? Been a while. If this is right I’ll come back with a quote.
Okay, I checked. I was right. So here’s mine:
Just a thought? Poker. Not your game.
Oz, to Giles, in Beauty and the Beasts
So neither of you is pregnant, failing or under indictment?
Joyce, to both Buffy and Dawn in No Place Like Home.
“Thank you, Dragnet.”
Andrew to Jonathan. They’re both in jail following the robbery that was foiled by Buffy breaking Warren’s balls, after which Warren shot Buffy and accidentally also shot and killed Tara. “Seeing Red.”
I shall reward you. Only one of your women shall die, and you shall be the one to choose!
That’d be a little insane troll logic from Olaf the Troll in his first appearance.
“Is that the harlot?”
Joyce to Buffy. No, to Faith. 
Joyce to BF in Who Are You?
“I swear on my mother’s grave… should something fatal happen to her, God forbid.”