Thin Mints, also known as cookies laced with the equivalent of crack cocaine, are my favorite kind of Girl Scout cookie. To be sure, I do like some of the other Girl Scout cookies, such as the coconut/caramel Samoas and the peanut butter and chocolatey Tagalongs. If I was only allowed to get one box, though, it would be Thin Mints.
There's something to be said about a simple cookie, part dark chocolate, part mint that is a can't miss combination. In the candy market, York Peppermint Patties and Junior Mints utilize the same combination with great success. Thin Mints go one step further, though by being highly dunkable in milk, which allows for a nice balance to the bite and sweetness of the mint against the dark chocolate.
I've also heard (though I haven't done this myself yet) that Thin Mints freeze well, so you can get your Girl Scout cookie fix later on in the year, long after the swarms of elementary school aged girls grace the entrances to grocery stores and supermarkets. To my way of thinking, that just seems like you're not eating your Thin Mints fast enough. :-D Though I suppose to someone who likes Girl Scout cookies as much as I like Cadbury Creme Eggs (and mourns their loss after their season passes just as much), perhaps stocking up and freezing a supply isn't that bad of an idea.
All I know is, when it starts getting to fall/winter time, it's time to be on the lookout for Girl Scout cookie pushers. The Thin Mints are delicious, are pretty inexpensive still, and support a great organization. Box of Thin Mints, please.