In order of hardest to easiest, FAR, REG, AUD, and BEC. My reasons for this ranking is the amount of material to study, opportunity to be exposed to the material in school, and the style of the questions on the CPA exams. Seeing everybody's rankings is fascinating, because in my conversations, most people agree that FAR is the hardest with REG a close second and BEC is the easiest.
FAR is the hardest, because it's virtually 4-5 semesters of college (undergrad and grad at my uni). In Becker, it has the most chapters with 10 chapters (twice as thick as BEC). It's a mix of a lot of granular detail, with robust problem solving techniques, every possible JE, and you have to know US GAAP, IFRS, NFP and govt accounting conventions. Most candidates don't take governmental/NFP accounting in college. Plus, many people aren't exposed to M&A and international accounting issues either like derivatives and foreign currency.
REG is up there with FAR, because it's about 3-4 semesters of college (undergrad and grad at my uni). In Becker, it has 8 chapters with 4 of them being nothing but rules and laws to remember. There is a lot of detailed facts (ie. phase-out thresholds, exemption amounts, etc.) that need to be used in the problem solving techniques for personal and corporate/partnership taxation.
AUD is challenging, because it's all conceptual and memorization. It's only one semester at my uni, but my uni audit course didn't dive into private audits and governmental audits, so, that's new material. In Becker, AUD is 6 long chapters. I think what makes it hard is that you either know the answer or you don't. Plus, it does have the lowest pass rate. I'd say if AUD had more questions forcing test takers to actually audit, it could be very hard.
BEC could be the hardest, but they don't dive too deep into any of the various topics. BEC is a hodgepodge of junior level undergrad classes. In Becker, BEC is the skinniest book with 6 short chapters for the most part. Plus, there are no Sims. Writing isn't hard for most aspiring CPAs. There are a lot of formulas, and the cost accounting, finance, and economic material can be a lot.
BEC - 85
AUD - 81
REG - 84
FAR - 7/24/16