I dont have a lot of their stuff but out of the older Mac albums I have (Bare Trees, Future Games, Mystery To Me) it would have to be Bob. He wrote SO many great songs on those albums, and his two from Bare Trees are the best ones on the album. I also love the Yessy "Future Games" the rocking "The City" and the melancholy pop of "Emerald Eyes." "Hypnotised" really should have been their first hit, thats one of the coolest drum beats ever.
Christine I think should be up for this too. She appeared on 8 albums (Im counting her uncredited performances on Kiln House and Then Play On) before Rumours so I think she's overqualified for "best Pre-Rumours member!" She wrote some of her best songs back then!