For me, REG was the most difficult since I had no previous experience in US taxes. BEC is tricky too.. too much sections to be tested.

FAR is too long...but not so difficult.

Good luck!

Now see, I thought BEC ended up being the hardest for me--maybe because I went into it thinking it'd be the easiest with only 90 mcqs and no sims.  HA.  It took me 5 times to pass. 

I originally thought FARE would be the the beast, but that one I passed on the first shot.  Maybe because I went into it giving it 110% knowing that it was supposed to be the worst.  Still, I think I figured out how to amortize a bond the night before the exam.  LOL

REG wasn't too bad.  The second time I took it, I felt pretty prepared considering I only studied for about a month.

And I took Auditing so long ago now that I don't even remember what was tested, but I passed with a 75.

Probably the best advice I could give would be not to procrastinate.  I sat on my audit score for so long that I only had 3 months to pass 3 exams.  I don't recommend this!

Hardest AUD -REG-FAR-BEC

but then u have to remember that people knowledge and exp plays a big role in that too.