Some quotes:
Elena: What happened?
Damon: Katherine happened.
Elena: Katherine was in this house, that means that she's been invited in. What are we gonna do?
Damon: Move.
Damon: To risk another frown line encroaching on a very crowded forehead. We... kissed.
Elena: And you thought it was me?
Stefan: What do you mean you kissed?
Damon: Well, you know when two lips pucker and they go [Damon makes kissing sound]
Damon: Stake her, rip her head off... something poetic. We'll see.
Elena: How did you leave it in there with John?
Stefan: I... asked him to leave town.
Elena: Asked? You threatened him.
Stefan: Yeah, I threatened him.
Stefan: What game are you playing?
Katherine: Why, you want to play with me?
Stefan: I don't know. How can I play if I don't know the rules?
Katherine: No rules, Stefan. Don't you remember? No rules.
Elena: I really wanna know how you're doing.
Damon: I kissed you.Thought you kissed me back. Doppelganger high jinx ensued. How do you think I'm doing?
Damon: Why is it such a surprise that I would kiss you?
Elena: That's not a surprise. I'm surprised that you thought I'd kiss you back.
Damon: Now I'm hurt.
Stefan: Don't flirt with me Katherine. I'm not Damon. I haven't spent a hundred and forty five years obsessed with you.
Katherine: Yeah, based on your choice of women, I'd say otherwise.
Damon: I kissed Elena.
Stefan: Because you feel something for her! Because you actually care. And I'm not gonna let Katherine come in here and destroy that part of you that is finally, after all of this time, willing to feel something. She'll try to break you. She'll try to break us. And how we respond to that will define us. It's our choice.
Damon: I have a question. Answer it and it's back to fireworks and rockets red glare. Answer it right and I'll forget the last hundred and fortyfive years I've spent missing you. I'll forget it. How much I loved you, I'll forget everything and we can start over. This can be our defining moment. We have time, that's the beauty of eternity. I just need the truth, just once.
Katherine [to Damon]: Stop. I already know your question and its answer. The truth is I've never loved you. It was always Stefan.
Elena: I care about you, I do. But I love Stefan, it's always gonna be Stefan.
Katherine: Game On.