I gotta say Biochemistry. However, that's probably because I was a biochemistry major and therefore inundated with it. Biochem is so fundamental to medicine - sadly, it is often taught badly at many institutions. I was lucky enough to have a lot of very good professors.
All the inner workings of the body come down to molecules. You can't be a good doctor with only a basic understanding of macroscopic signs and whatever drug Plumb's tells you to give.
Physiology was made easier because I could understand the molecular processes behind it e.g. the gradients in the kidney that keep things filtering correctly. Pharm too - knowing what type of drug it was meant I had an idea of the structure, and then could guess how it acted even if I didn't know. Same with anesthesia. Same with nutrition. And don't get me started on clin path - biochem was so helpful for that I can't even describe!
In more general terms, biochem taught me to be able to think in a logical, stepwise pattern. We have this molecule and this molecule, if we combine them what do we make? If we change this group here, how does it affect the structure? This type of thinking was essential for me to be a good diagnostician. Ok, we have this clinical sign and this lab value - how can we work this out? What could be causing this? Ok, we gave this drug and this has improved but not this - what is happening on a cellular level here that we need to address?