It's probably already been mentioned, but Masquerade is my most used item, hands down.
I love it because it lets me change my mouth to go with what I'm wearing, which makes more of a difference than you'd expect! I have two now so that I can make a more smokey eye look if I want to (like now). Seriously, for all the fuss made over the Agape Scissors on another thread (and here), Masquerade has by far been my best investment, and I bought it with gold before it's price shot up! ^.^
Masquerade. (Buy it. Wear it. Love it.)
Note: I've read the entire thread and Masquerade has been mentioned a lot, probably more than any other item.