I prefer unit 3. I guess the equilibrium stuff in unit 4 is interesting, and I enjoyed chapters 15-17. However I hated ch 18. Production of Ammonia/Sulfuric acid was somewhat interesting, however I feel as if I personally just 'read' it and noted stuff down more than really taking the time to understand everything fully. I also liked thermochemical equations (ch 25) as I guess its mathsy
Galvanic Cells are somewhat interesting as well - I remember my tutor telling me about all this electrochemical series stuff last year! I guess it's good to know that that knowledge wasn't wasted. Didn't like reading through the cells/batteries chapter, as it was so theory-heavy.
The final chapter is so-so. The theory is kinda ehhh but there are calculations involved (YAY FARADAY

)
Just my two cents.