B

The key of the month will be B.  I first make sure I know the scale itself. Then I make sure I have in hand the 9 chords associated with the key (7 plus the tritone substitution, and the dominant chords for the I and IV to play the blues). I then compose a piece in the key. (I have done this, but haven't written out the "B" section yet.)

I can do a key a month this way, always choosing as my next key the one I like or know the least.  So Feb will have to be E or Gb. By the end of the year, I can simply review the keys I already have a better grasp of.

Writing in B is not hard per se, but it led me to an idea I would not have had otherwise: iii IV7 iii ii / iii IV7 II7 V (tritone sub) / I vii I V I...  In other words, it is not just the completist urge to learn all the keys, but the ability to generate ideas out of new ways of thinking about familiar relationships.