My favorite has got to be the "Potential" speach:
XANDER: (walks into the room) Aw, I'm just thinking about the girls. It's a harsh gig, being a potential. Just being picked out of a crowd, danger, destiny, (grins) plus if you act now, death.
DAWN: They can handle it.
XANDER: Yeah. (sits in a chair in front of the desk) They're special, no doubt. The amazing thing is, not one of them will ever know, not even Buffy.
DAWN: Know what?
XANDER: How much harder it is for the rest of us.
DAWN: No way. They've got"”
XANDER: Seven years, Dawn. Working with the slayer. Seeing my friends get more and more powerful. A witch. A demon. Hell, I could fit Oz in my shaving kit, but come a full moon, he had a wolfy mojo not to be messed with. Powerful. All of them. And I'm the guy who fixes the windows.
DAWN: Well, you had that sexy army training for a while, and"”and the windows really did need fixing.
XANDER: I saw what you did last night.
DAWN: Yeah, I"”I guess I kinda lost my head when I thought I was the slayer.
XANDER: You thought you were all special. Miss Sunnydale 2003. And the minute you found out you weren't, you handed the crown to Amanda without a moment's pause. You gave her your power.
DAWN: The power wasn't mine.
XANDER: They'll never know how tough it is, Dawnie, to be the one who isn't chosen. To live so near to the spotlight and never step in it. But I know. I see more than anybody realizes because nobody's watching me. I saw you last night. I see you working here today. You're not special. You're extraordinary.
DAWN: Maybe that's your power.
XANDER: What?
DAWN: Seeing. Knowing.
XANDER: Maybe it is. Maybe I should get a cape.
DAWN: Cape is good.
XANDER: Yeah.