Popcorn with real butter.
Seriously, this is probably the single best snack out there in the Universe. The popcorn is dirt cheap, and though the butter tends to be more expensive than margarine, it is worth it to stock up when there are sales on butter so that you can enjoy it's loveliness on warm popcorn straight from the popper. Margarine makes your popcorn soggy, butter doesn't. I don't know the scientific magic involved, but it is true.
Popcorn is truly perfect. It is cheap (like I said), not fattening, crunchy, tasty... I love it. It works for breakfast, lunch, supper, snack time, movie watching, reading a book, playing on the 'puter.... it is an all purpose snack. The only thing is the greasy fingers you get from the butter. A couple of paper towels will fix that right up, though...
I love popcorn!