For American candy: Milky Way Dark (I normally don't care much for dark chocolate, but the mix is just perfect with this one.). For non-chocolate yumminess, Spree. (I also like Skor bars, and I actually don't seem to have any trouble finding them where I live, more's the pity, because I really need to lose some weight!)
For UK candy, Terry's Chocolate Orange. (I *love* chocolate mixed with orange; I can eat the World Market Chocolate-Orange straw cookies by the can-full if you don't stop me! They are especially wonderful crumbled on good vanilla ice cream.)
Honorable mention to McVitie's Penguins, though those are really more of a cookie than a candy. My children adore them, so much so that they always demand them on birthdays and Christmas. (I can only find them at World Market, and they are terribly expensive in the US. Whenever I travel to visit relatives I stock up on them, because they are dirt cheap in the UK and Ireland.)
PS: I know I'm not supposed to say so, but my favorite Frango flavor is Toffee. I used to love the original mint, but I can't really enjoy mint anymore; tummy issues. Most Macy's have some Frango available in stores, but if you want the full range of product, you still need to visit one in Chicago. (Macy's actually sold the Frango brand to Garrett Popcorn a few months ago, though they don't intend to carry the chocolate in the popcorn shops. Many people are hoping that they will move production back to Chicago. As of now, Macy's still plans to sell Frango in store.)