Lily Blossom is pretty hard to find now, Blossomforth is sort-after, I've seen people looking for Feathermay quite a lot as well with no luck finding her...
FS Applejack, Rarity and Fluttershy are seemingly the hardest FS' to find, I've seen a lot of people complaining that they can't find them and I've only seen Rarity FS once :c
I'd say one not to bait would be Diamon Rose and Cherry Pie... many of us could not find them in our areas
They're really common where I live O: or were. I once walked into my local Entertainer and they had more Cherry Pie and Diamond Roses' than any other pony put together, but because of the Entertainer £3 sale, they're almost all gone now; but they were around for at least 4 months, and there's still a few on my local Toys R Us... o _ o