In all states, the Series 6 will be your first exam. In most states, the 6&63 will both be needed to really be able to use the Series 6.
After you take both the 6 and 63, the next test gets trickier. It should be started soon after, as the laws are the same, and the material can be similar, the closer you take them, the easier they can be.
The Series 65 is the Investment Adviser Representative Exam or the Fiduciary Exam. It allows the sales of managed accounts and fee-based management. For people who are primarily AGENTS, the Series 65 is the most important of the two. For people who are primarily MANAGERS, then the Series 26 is vastly more important.
The Series 26 is the Limited Principal Exam and allows supervision of Series 6 reps. For those in management and supervision this license is mandatory, and for some of those, with large teams that rarely write on their own pen, the Series 65 isn’t even optional or suggested; it just isn’t needed.
The Series 63 and 65 are similar, while the Series 6 and 26 are more similar to each other than any other exam. That is not to say they are the same, merely similar. The 65 is like the 63’s older brother, with a bad attitude, and a chip on his shoulder. The 26 is like the 6’s distant cousin, who although they may look alike in certain ways, speaks a different language, and just doesn’t like you.
The 65 and the 26 are both a step above in difficulty, complexity, and time required to master, then the 6 and 63.