This is an impossible task. I commend those who have tried to accomplish this task.
The hard part for me is that some of my all time favorite Mac songs are on the albums I don't like. For example, Nights In Estoril is one of my all time favorite Christine Mac songs. But its on an album that I don't care for. I would probably put Mystery to Me pretty high because of Why, Just Crazy Love, and Hypnotized. Heroes are Hard To Find would also be pretty high. Chris is just ready to explode with her hits and she writes some amazing stuff like Come A Little Bit Closer, Bad Loser, and Prove your Love. Bare Trees is a great album too.
I am biased because while I like the early Mac stuff, I just think Christine added so much to the band.
Rumours of course is amazing and probably on everyone's top list but it would battle Tusk IMHO for the top notch. I also LOVE Mirage. Yes its safe and more pop rock but its so classic Fleetwood Mac that I love it. The Fleetwood Mac album is also amazing . Tango sold extremely well and had lots of hits but other than the hits and Isn't It Midnight, the album is too 80's synth for me. Thank goodness they did a Live album to capture the magic during their peak. So that album has a special place too. Say You Will is a very solid effort and is a great album. If Chris was on that album it would be near the top.
Sorry I chickened out on this assignment but its impossible for me. Some of my favorite tracks are on albums I am not crazy about. And some of the better albums don't have my all time favorite songs. Hope that makes sense