My absolute fav. quote is:

Willow: Great. I'll give Xander a call
. What's his number? Oh, yeah, 
1-800-I'm-Dating-A-Skanky-Ho.
Buffy: Meow!
Willow: Really? Thanks. I've never
 gotten a 'meow' before.
Buffy: Well-deserved.
Willow: Darn tootin'.
(Phases)

The second best Willow quote is:

Xander: Look, it's not a good idea.
Willow: There's no use arguing with
 me. Do you see my resolve face?
 You've seen it before. You know what it means.
(Becoming Two)

The third place award goes to:

Willow: Okay, somebody explain the whole
 'he will suck the world into Hell' thing, 
because that's the part I'm not loving.
(Becoming One)

And the bronze quote is:

Xander: Willow, come on!
Willow: I knew it! I knew it! Well, not 'knew it'
 in the sense of having the slightest idea, but
 I knew there was something I didn't know.
 You two were fighting way too much. It's not 
natural!
Xander: I know it's weird...
Willow: Weird? It's against all laws of God
 and Man! It's Cordelia! Remember? 
The, the 'We Hate Cordelia' club, of which
 you are the treasurer.
Xander: Look, I was gonna tell you.
Willow: Gee, what stopped you? Could it
 be shame?
Xander: All right, let's overreact, shall we?
Willow: But I'm...
Xander: Willow, we were just kissing. It
 doesn't mean that much.
Willow: No. It just means that you'd rather
 be with someone you hate than be with me.

(Sorry, I couldn't pick.  it was a tie)

Willow: Let's get this straight. I don't 
understand it, I don't wanna understand it, 
you have gross emotional problems, and 
things are not okay between us. But what's
 happening right now is more important than
 that.
Xander: Okay.
(Innocence for both of them)

Some other good ones are:

Oz: I'm gonna ask you to go out with me 
tomorrow night. And I'm kinda nervous 
about it, actually.
Willow: Oh. Well, if it helps at all, I'm gonna
 say yes.
Oz: Yeah, it helps. It-it creates a comfort
 zone. Do you wanna go out with me
 tomorrow night?
Willow: [cringes and slaps her hand to
 her forehead] Oh! I can't!
Oz: Well, see, I like that you're unpredictable.
Willow: Oh, it's just it's Buffy's birthday, 
and we're throwing her a surprise party.
Oz: It's okay.
Willow: But you could come. If you want 
to....you could be m... my date.
Oz: All right. I'm in. 
Willow: [to herself] I said 'date'.
(Surprises)

Buffy: Go ahead. You know you wanna say it.
Willow: My boyfriend's in the band!
Amy: Cool. 
Buffy: I think you've now told everybody.
Willow: Only in this hemisphere. 
(Bewitched, Bothered, and
Bewildered)

Willow: No, Xander's right! My God, you
 people are all... Well, I'm upset, and I
 can't think of a mean word right now, but 
that's what you are, and we're going to the
 factory!
Xander: Yeah!
(Innocence)

Buffy: Well, I think it's nice that he's not
 just being an animal.
Willow: It is nice. He's great. We have a
 lot of fun. But I want smoochies!
Buffy: Have you dropped any hints?
Willow: I've dropped anvils.
Buffy: Ah, he'll come around. What guy 
could resist your wily, Willow charms?
Willow: At last count, all of them. Maybe
 more.
Buffy: Well, none of them know a thing. 
They all get an 'F' in Willow.
Willow: But I want Oz to get an 'A', and, 
oh, one of those gold stars.
(Phases)