Out of any type of science that you have studied, what is the most challenging... is inherently complicated as carbon is tetravalent.
Well, I have a degree in physics with diverse interest, so I'll toss my opinion: bronze medal would go to topology/general relativity. Always treading on mathematics, only sometimes achieving tantalizing "direct" insight. Silver medal would go to organic chemistry. I took a course in it and ever since, I have respect for the field : much to understand and a LOT to learn - quite hard I you got crappy memory like I do. Gold? Logic. One summer I've decided to thoroughly go through famous Godel's theorems. A painful experience, sense of misplacement, exhausting and blinding thoughts but with feeling of deep satisfaction at the end.
Do note that I'm not very good at any of these fields, so take this cum grano salis, just as subjective judgement.
Though I have not studied physics formally, physicists have told me that most of it is just applied math, which I find to be very easy.
There are no much physicist to agree with this: my professor in elementary particles was known to say, when explaining some some tedious mathematical parts - now, you can turn off your brains, this is only mathematics.
Of course, that was in humoristic vain, but for sure - physics is not only applied mathematics. If it was, then mathematicians would regularly collect Nobel prizes in physics, just as they do in economics.