I have a lot, but the only one coming to mind right now is Buffy's speech from the episode "Bring on the Night" in season seven. I can't find a clip of it, but here it is:
"I'm beyond tired. I'm beyond scared. I'm standing on the mouth of Hell and it is going to swallow me whole.
And it'll choke on me.
We're not ready? They're not ready. They think we're gonna wait for the end to come, like we always do. I'm done waiting. They want an apocalypse? Well, we'll give 'em one. Anyone else who wants to run, do it now, 'cause we just became an army. We just declared war. From now on, we won't just face our worst fears, we will seek them out. We will find them, and cut out their hearts, one by one, until the First shows itself for what it really is. And I'll kill it myself. There is only one thing on this earth more powerful than evil. And that's us. Any questions?"