I have two favourites: Fafi and Cult of Cherry. Fafi, because the colours were great and also because that collection jump-started my MAC addiction (it had been a casual attachment before, haha).
Cult of Cherry further deepened my obsession; it was how I found this amazing site
and was the first collection where I wanted more than maybe two or three lipcolours; hell, I wanted everything at first! Then I found some permanent collection dupes (Syrup l/s and Pop Mode l/g for Lightly Ripe and Jampacked), and ended up getting five of the seven items I was lemming.
I have no regrets about my newfound obsession, rather, I welcome it with open arms.