My love, of course, is the Alpha Phi badge (particularly, the one with pearls and a diamond in the center). The replicas of the original
"lazy phi" pins with opals are incredibly unique and beautiful too.
If I were choosing another sorority's pin though, I think I would have to go with the
Alpha Omicron Pi badge, which is extremely pretty and unusual (in a very good way). I've never seen one in person though because the universities I attended had no AOPi chapters.

I think I have a better chance now that I am in Georgia though (I see AOPi stickers everywhere out here!).
In fact, I just love the
badges that are stacked mongrams of the sorority's letters (Alpha Gam, Chi Omega, Alpha Omicron Pi, Gamma Phi Beta, & co.).
It's hard to pick just one because, truly, they are all very unique and interesting in their own ways, and each special to their members as my badge is to me.