I'd say physics, simply because chemistry is reducible to physics, and biology is reducible in theory to chemistry, cosmology is reducible to physics, astrology is reducible to physics, etc.
However, that being said, many people might also conclude that culture is reducible to biology, thus reducible to physics. I think that's making a stretch. Reductionism is good, greedy reductionism is bad. That's why I put the
in theory tag on the reduction of biology to chemistry. Even though it is possible in theory to reduce it ultimately to physics, that is just not the right level at which to study the field in a meaningful way. Language, Art, and other aspects of culture are emergent phenomena. It would be silly to try to reduce behavioral science to physics (although physics might provide some insight at certain points, or for certain questions).
Still, although there are some emergent sciences, and greedy reductionism is bad, you can't deny physics as being the cornerstone of the whole shebang.
Philosophy is its own monstor, emergent and ugly.

I actually like philosophy quite a bit, but I can't see it being more important than physics. Without physics, many other sciences would be barren. Without those sciences, the philosophy of science would hardly be interesting.