Dreams or Go Your Own Way.
Both songs are perfection. As Chris put it, there was a parting of the ways musically among the band members (at least the three songwriters) after Rumours, so that album was much more of a unified, collaborative effort than Tusk, Mirage, Tango, or even the white album (which was more of a "getting to know you" album than anything). I think they were all synched up beautifully on Rumours which adds immensely to its charm. Everyone was on the same page musically even though they were at each other's throats personally---how ironic!
And the precious fact that these were Lindsey's and Stevie's songs to one another is not lost on me either.