If you want to start with the most difficult problems, the score sheet suggests that these are, in descending order of difficulty, A5, B6, B5, and A6.
At the other end of the difficulty range, A2, B1, and A1 are all hanging sliders right over the plate.
And if you're still hungry for Putnam-level math problems, there's one in Chapter 31 of Fire from the Sun. (It may actually be a Putnam problem; I forget now where I found it.)