This is a fun thread and its really hard. Its safe to say most albums have something for everyone. I like the early stuff but I fall in love when Christine joins. She is the special sauce in the Mac for sure. I agree with Mick that Bob Welch joining the band was just as profound as Stevie and Lindsey. Chris and Bob writing and harmonizing together and the band gets mystical. This was a pre-tense and laid the groundwork for Lindsey and Stevie. Chris really explodes with writing great songs. Stevie and Lindsey were magical components and the band started to embrace more mainstream pop/rock.
Can you imagine if the White Album or Rumours was the double album instead of Tusk? In any event for half a decade the band was on a seriously creative roll. I deeply respect and love Behind the Mask. Half the songs are excellent IMHO and the other half not so great. I don't care for Time but ironically contains one of my favorite Mac tunes "Nights in Estoril". (Go figure). It just proves there is a diamond rough just about everywhere. After Tusk there was just too much pressure on the band commercially. Mirage was a jerk reaction to Tusk and Tango was pressure to be successful after 5 years and Mick's bankruptcy. I think that is why I enjoy Behind the Mask and Say You Will. They are creative without the pressure to be a commercial success. Completely not pop driven and I love that. Its sad we never got one last effort from these 5. But we need to be thankful they are still touring and making new music in other ways.
Still I believe.....BACKSTABBING AND BETRAYAL will show us how