I have to go with the one where Dora and Marten kiss for the first time. At the time of that comic, I had literally just started a relationship like the day before it was posted, and I had started to think that it was closely following my life. I know it's odd, but what can I say?
Anyway, when I read that comic, I literally stood up and cheered. I don't know why, maybe because I see a lot of Marten in myself, or because I was stone cold hammered, but I loved it, and still do.
Though, the first comic where Dora and Marten have sex was awesome as well.