I think it was Amy. Because he was thinking about Amy that day, and not any of the other people whose death he's responsible for (or thinks he is; "what's dead should stay dead"—there's a reason John's in this poll); he was thinking about Jo when he saw the blonde bartender, I'm sure, and he's always thinking about Sam. Also because Osiris heard him talking about Amy and not about anyone else. But I'm delighted to hear arguments to the contrary.