Jacqueline Church presents a reasoned and, well, sensible look at the difficulty of choosing which foods are "best", and does it a million times better than I could.
One of her main points is that we should embrace incompetence. In realizing that the qualities of "good" food: local, organic, sustainable, (healthy, reasonably priced, ...) can often be in conflict with each other, and we shouldn't be looking for easy answers or a quick fix.
There's no point in me reiterating the whole article though, when you could be reading it right now. So go. Yes you. Git!