I would vote Mystery to Me as it truly is a Rumours before Rumours. No doubt that if the promotion could have been followed through, it would have at least sold in the White album category. It's held up well with the passage of time and contains great writing and performances by both Bob and Christine. Hypnotized and Keep on Going later went on to become staples in classic rock radio and back in the day easily could've been chart toppers.
__________________
|