Still finishing off my degree but since the beginning,
Hardest;
Physical; I've never really enjoyed Thermodynamics. They involve some partial differential calculus and even a few PDE's. I find Atkin's textbook heavy going. Photokinetics is also a very weird subject and I find a lot of it very unintuitive. Both of those are definitely the hardest subjects I've studied to date. Thermodynamics is fundamental to Chemistry however so got to keep plugging at it!
Organic; Nuclear Magnetic Resonance. Although I feel reasonably competant in the background theory and analysis of basic 1D 1H or 13C NMR, there's so much more to it like 'Nuclear Overhauser Spectroscopy', TOCSY and other 2D/3D stuff that I admit I do not feel strong on at all! It never seems to have an endpoint. It's very a difficult subject to explain to people in laymans terms also... The ''Robinson Annulation'' I will admit was demon to learn also. 'Protein Synthesis'/Chemical Biology I also find very ''memorise''' heavy...
Inorganic; Actually Periodicity isn't a language I can speak very well. For some it's very intuitive but I find I have to revise all the trends a lot... Other weird ligand trivia like the 'trans-effect' and some areas of catalysis are very strange indeed. Bioinorganic is interesting but I hate having to memorise Nucleobases and their bonding patterns. Materials is interesting but it seems like a very ''all over the place'' subject to me and nowhere near as 'organised' as parts of Physical or Organic.
Easiest;
Organic; I've always found most Synthetic Organic Chemistry and Stereochemistry in general very natural and like a language to me. I'm also enjoying the Woodward Hoffman laws and how they mix Quantum Theory to explain Pericyclic Chemistry. Challenging but not horrendous for me. Parts of NMR I actually don't find too challenging (once I understood how things like 'scalar coupling' arise from the different spin-states.) I think Organic is a more beautiful subject than people give it credit for...
Physical; Symmetry and Mathematical Group Theory I'm picking up quite well I feel, while I still haven't learned all of it, it doesn't feel too difficult. I like how you can (most of the time) visualise what Symmetry Operations are doing. Surprisingly I found Quantum Theory a lot less difficult than Thermodynamics and It's quite nice how orbitals can be treated as 'Symmetry objects', I would be lying if I said it was easy though. Kinetics is a nice subject and the calculus isn't too difficult in it, it also applies very nicely to other areas of Chemistry.
Inorganic; Ligand Field Theory and Crystal Field Theory are the most logical bits of Inorganic I've found and I like how they can explain so much, like Magnetism, Colour, Geometry etc Again I like the visual aspect of it and It's cool how you can 'prove' whether a complex will be paramagnetic or diamagnetic...It's a great theory. HSAB is also a nice theory to deploy when all else fails!
Most Enjoyable: Got to be a tie between Molecular Symmetry, Magnetism and Organic.
Interestingly rather similar to the above posts!