Season: 4 (The Chrismukk-huh?)
This was the episode in which Taylor and Ryan fall off a ladder and wake up in an alternate universe. When Ryan sees how different life would be without him, he realizes Marissa would have died in Tijuana if not for him. Sure, the episode helps him let go of Marissa's tragic death, but she never should have died in the first place. She wasn't even that great a character but it was nonetheless the show's demise.
*It would also be hard to rank moments from this episode since the events technically aren't real.