Here's my opinion from hardest to easiest:

FAR, AUD, BEC, REG

Of course I think this would be different for each individual depending on your background. FAR just has a ton of material to learn and requires a ton of study time, and AUD requires a lot of memorizing.  

I think we all can agree that FAR definitely has the most material ... and for some reason I felt like it also had very little continuity between chapters. For example, REG I found all the tax chapters to flow together very nicely with concepts repeated (same with law) but FAR covers soooo many topics that that flow never seems to happen. FAR was the one exam I walked out of feeling like I got completely screwed because of the simulations... Also, I didn't think 4 hrs was enough time. I'm still trying to understand why audit is a half hour longer then FAR and covers like 1/2 the material FAR does. However, that's likely because I was an auditor for a year ... I remember Audit in college and it was like a foreign language. Auditing is very hard to understand in theory unless you have done it before for work (then it makes perfect sense).

This being said, I think that if you are a tax person AUD will be the hardest section for you, since it'll be very different then what you are used to. Being an auditor I was expecting REG to be my hardest section b/c of the tax, but I found the tax in the CPA to be rather straightforward memorization ... so it wasn't terrible. Also, it helped that I've always had a knack for law (almost took the LSATs a few yrs ago) on it as well.

Thus with all experiences aside I believe that FAR is the hardest exam from just a volume of info standpoint. REG/AUD could be the hardest for you based on your background, but all else equal FAR is the most intense. However, I think it is harder for a tax person to take AUD then a auditor to take REG b/c I feel REG is mostly memorization and AUD you cannot just memorize, you need to understand completely.

In the end FAR was my hardest exam, but also the one I went into feeling the most prepared. The hardest part about FAR is that a lot of the MCQs have calculations in them, so they take a lot of time. Thus, that is the exam I ran out of time on ... In my opinion they need to make FAR a 4.5 hr exam, but w/e. REG involves a lot of numbers, but the calculations are all addition and subtraction (the trick is knowing what given numbers to add and subtract) so it is much faster the FAR (more complex calcs). AUD is all theory so if you know your stuff you'll finish fast and pass, and if you don't you'll take forever and fail (i see this section as the most stark "you either know it or you dont" section). BEC is a mix of theory and calculations .... personally I thought BEC was very easy and walked out of the testing center knowing I had passed ... but apparently there are a lot of other people who have taken it multiple times.

One thing is for sure though.... if you are an auditor your easiest section will be AUD. If you are a tax person your easiest section will be REG. Then FAR and BEC are wildcards....they may be hard, they may not.

So those are my thoughts :-)

Hardest

FAR - BY FARRRRRR

REG

BEC

AUD

Least Hardest

I spent more time on FAR than the other 3 combined it felt like.