I am hopeless at memorising quotes and was looking for a particular one; Google drew a blank (funny how it often has everything but what I'm trying to look up), so I plumped for this one instead, from Episode 6x22: Grave.
“I love you, Willow.... So if you’re gonna end the world, you’re gonna have to start with me.”
This is one of the few times that Xander is the hero for the day, this quote really sums up his relationship with Willow, and I love in this episode how after Xander annoyed me a lot in Season 6, he makes up for things by ending Willow's downward spiral, and stopping her from ending the world.