CFA exam candidates who take the level 1 exam often indicate that the FI, derivatives, and FSA (or FRA) * are the most difficult. I took the exam in December and found that SS9 was by far the most difficult subject to study (deferred taxes and leasing were difficult to understand). However, the questions I found most difficult on the exam were those about ethics. I could usually narrow it down to 2 options, but I was almost never sure I had chosen the right answer. The best way to study for the CFA Level I exam is to follow the CFA program curriculum and have enough time to learn the material with the ideal learning style.
The real debate here is between CFA level 2 and level 3, since deciding which is the most difficult CFA level between these two exams is less clear. For CFA certificate holders, I think there's general consensus that the CFA Level 1 is the easiest of the 3 levels of the CFA exam. One of the most important ways to prepare for the CFA Level I exam is to have enough time to study. From an objective perspective, the difficulty of level 3 of the CFA is equivalent to that of a master's degree, while level 2 of the CFA is equivalent to a degree program. If you are interested in taking the CFA Level I exam, check the CFA Level I exam requirements to see if you are prepared to enroll.