Whatever your thoughts are on the show, it's quite obvious that Chase intended 6b and 6a to be the best. The difference between 6 and the rest is like comparing algebra at the high school level to a college level course -- one is relatively an overview of the subject, while the other continues the mode of thought in it's naturally progressive state -- and that's what we see with Chase's arc as a writer. He and the show seek to be a progressively intricate, complicated, contemplative work, one that's always improving and evolving...Such is the nature of life, such is the nature of art. The same can be said about the character arcs in the show.

6b was intended as Chase's crowning piece, followed by the penultimate 6a, while 2 and 3 represent the classic, vintage era of the show when it first took off with American audiences and was at it's peak in terms of viewers. Of course, there's no telling where the show might have gone had "Lydia" not died.