I would have to say my favorite monthly collectible would be the Techno-Mage collectible. Although not my favorite over all collectible, i think that the idea of combining science, along with its counterpart, magic, is quite interesting.
The idea combines two different parts of a spectrum, one logic, the other, superstition, and combines the two together beautifully, creating an item that fans of one or the other spectrum, or both, can enjoy.