Knowing fully well that this seems like I am taking the easy way out, I must confess that my favorite is Mwee the Dragon.
Before you dismiss me as a lazy good-for-nothing, hear me out. Many of the items on gaia are primarily designed for one specific gender or the other. It isn't intentional or bad, it simple is. Mwee escapes that by being neither feminine, nor cutesy, nor masculine. Of course, there are other items like this, such as the Kong Sang Scarf or Coco the Kitty Plushie, but no other item also encompasses the spirit of October quite as well. Dark colored and slightly haughty looking, Mwee could be incorporated into almost any Halloween costume, or into a casual everyday avatar. She also fits in with the idea of fall being a spiritual season, in which the dead and the fairies come out to play with the living.
Beyond all that, Mwee is a dragon, and dragons are awesome.
So, I say Mwee the Dragon is the best October monthly collectible, so far.