I'll give you one better than ranking them–my overall experience.
FAR-studied the longest; lots of material to digest; IFRS, Govt. Accounting, not-for-profit, and GAAP can become confusing learning together because their rules are different in certain circumstances and similar in others; had about 7 minutes to spare when I finished the test; I was the most stressed on this test and thought I failed it.
BEC-alot of formulas to remember, but as long as you write well, the written communications may help bolster your grade
AUD-the material was hard to study because it was by far the driest of them all. MCQ's can be tricky. Most relaxed when taking this test. Very few formulas to memorize.
REG-Business Law is easy; remembering all the rules within rules within rules of tax accounting can be confusing to keep it straight in your head. Phaseouts, floors, corp vs individual taxation can be challenging to memorize.
From hardest to easiest: FAR,REG,BEC, AUD. Although REG and BEC were definitely a close tie.
FAR 82
BEC 82
AUD 93
REG 87