Easiest to hardest: AUD, REG, BEC, FAR
Order I took them: AUD, BEC, REG, FAR
If I had to do it again? FAR, BEC, REG, AUD
I really wish I had gotten FAR out of the way... Studying for my first section was more detailed and focused since I wasn't burned out from constant study from the rest of the sections. But it might also be a good idea to knock an easier one out of the way first for a confidence boost and more knowledge of the testing process / what to expect.
A lot of people have difficulty with BEC but I didn't think it was too bad, probably because I'm fresh out of school and a lot of the topics were more fresh in my mind, especially the business/econ related topics. Plus most of the IT topics were second nature to me. And not having sims made it a lot less scary :)
Same goes for AUD, a lot of it is academic and as long as you know the concepts you can usually come up with a pretty good educated guess, even for really tricky/wordy questions
I underestimated REG since I am a tax girl, so I didn't put as much effort into studying as I should have. I failed the first time, but then did the Yaeger cram and passed.