For me physics and astronomy are in a dead heat for first.
I stand in awe at the idea of how unimaginably large the whole of the universe is (if in fact it does end. who really know though.) and how small I am in comparison. point for astronomy.
On the other hand the idea that all things in existence can be explained by one overarching theory of everything (Physic's holy grail, M-Theory) intrigues me and makes me proud to be part of a species that has the capacity and understanding to actually ask the question: "How and why does reality work the way it does?" Point for physics.
All branches of science are interesting in their own ways (Psychology is probably a close second in this little race I've got going.) but those two are my favorites.