A topic I can really sink my teeth into!
Go For It has been my favorite for years, because it's not too dark and not too light and has plenty of blue AND grey (love that touch of grey).
Years ago, the BEST MAC purple I owned was Epic! It's a very grey, dirty lilac - on the darker side of mid-tone. I still have my tube from the 90's but it's only to look at. 'Wouldn't want to put it on my lips anymore, sad to say. All this 90's style is coming back, so I am waiting for a new color along these lines already...
One purple I don't like is Smoked Purple - it's just too dry and dingy. I'd take Cyber over it any day.
Dominate is amazing - the metallic frost finish is so beautiful beyond belief.
Here's my purple collection (off the top of my head):
Go For It
Dominate
Strong Woman
Heroine
Riri Boy
Yung Rapunxel
Fervent
Boyfriend Stealer
Quite Cute
Blooming Lovely
Bust Out
Hellraiser
Cyber
Pervette
Epic
Sweet Succulence
Talk That Talk
Prince Noir
Lavender Whip
Feed the Senses
Till Tomorrow
Nocturnal Instinct mattene
Seductive Intent mattene
Fresh Amour mattene
Potent Fig mattene
Evening Stroll
All My Purple Life
Narcissus
Butterfly Party
Perky
So This Season
Feelin' So Good
Funtabulous
I <3 U
Fab Frenzy
Ban This!
Flaunting It
I don't think I'd count stuff like Playtime, Pink Popcorn or Petals & Peacocks - too fuchsia. Feel My Pulse is right on the edge. I wouldn't count Musky Amethyst or Caliente either - too berry (love those, though).
Punk Couture is going to be back up city for me and is probably going to shoot into my top 3 with Go For It and Dominate. Instigator too! One of these days, I will brave trying to order the Melt ones too, but I'd much rather try before buying, as always.