I didn't get into MAC until this year. I'm not a makeup person at heart, and I rarely use more than a bit of eyeshadow and lipgloss on a daily basis. I don't enjoy foundation (I rarely use it), I really despise blush (sorry blush ladies but I'm just not comfortable with it), or even mascara (it sucks but it's difficult for me to wear because I wear contacts).
I was sick of using Clinique and Dior, and a friend of mine let me borrow a few MAC shadows, and I fell in love. I found MAC's general colour payoff was miles better than Clinique, had better staying-power than Dior, and what has to be the largest range of eyeshadows I'd ever come across. So when I asked about 'favourite' collections, I guess I was looking to see what I'd missed over the years - the ones in recent years that stood out among the rest. The first set of MAC makeup I bought was a set of 11 pigments for $200 on eBay, and after authenticating they were real, I realised that my absolute favourite colour was Goldenaire. It was the perfect colour. Pink Opal was the first I tried, and it came a close second. I did a bit of research and found that both were from the Lingerie collection, and after looking at other items from the collection, I found I was disappointed I'd missed out big time. The colour range is exactly what I prefer to use on a daily basis - a bit of a shimmer, glowy colours, easy to blend, etc.
I think I skipped Barbie because I didn't really like the colours, but I also have a grudge against Barbie. *ahem*
(Yes, I'm one of *those* girls.)
Looking through the Color Stories, I think I would've loved the Rockocco and Rococo range, the Rebelrock range, everything in the Gold Play range, and some of the now-discontinued shadesticks from Points of Hue. Maybe MAC will re-release some of them. Personally I'd love to own a Stereo Rose, and a Petticoat (but not for more than $50 each!).
*Edit* I should say I like using a bit of powder rather than foundation, just to control oil and to give a glow. Hence, I like the MSFs.