My first thought was black coffee, although of course it's not food anyhow. At least I enjoy the coffee after adding cream and honey; tea, on the other hand, tastes awful to me even when sweetened.
I don't like oysters AT ALL and have always refused to eat anything with that flavor. I don't like licorice either, although the "fresh" licorice taste of fennel works just fine for me.
Otherwise, I wish I were a little more picky because my general problem with food is that I like it too much.
EDIT: Oh, I forgot what may be my strongest Yuck!(see, I wiped them from my mind.) For me, avocado has the worst taste of anything. I've tried it whole and in dip form and there's nothing you can mix it with that allows me to call it edible.