Time travel is a winning episode (and there are many in this list). Using the classic Christmas Carol trope, Death shows Peter what his life would be like if he did things differently.
Brian and Brian go back in time to 1984, but this year he drops Lois at the prom and changes his mind. Al Gore is the President. And shock horror! Lois and Quagmire have been married. All the children have his chin.
This episode has the best part: it becomes a hilarious parody of Back to the Future. It was also an example Family Guy's "Road to ..."" episodes. (Prepare for many more) - but this time with Peter and Brian instead of Stewie.