I'm definitely biased, but I'd have to say that my favorite quote in all of Homestuck has to be the entire conversation between Dirk and Hal right before Arquius, the bane of my existence, is created. I'd put the whole conversation here if I could, but here's my favorite excerpt.

Seriously. Good lord. This entire conversation is RIFE with thematic bullshit that I didn't catch until a second readthrough. Highly recommend rereading this conversation with the context of Dirk Strider as a character. My favorite thing to point out is how that last line shows not only just how bad Dirk's mental state is right now, but also how much Hal has diverged from him as a person. I'm not gonna rant about it too much because I don't feel like clogging the thread but it is such a good conversation.