The Shadow Spirit is one of the cooler items on gaia even still, with tons of options that make it a good fit for just about any avatar. Plus, the pixie has made possible Link avatars that look actually somewhat awesome. mrgreen
My all time favorite Monthly Collectible was the Demonic Pitchfork because it was out when first join Gaia and I saved up gaia gold to get my hands on that pitchfork and now I still have it with me and it was because of that demonic item that I started to collect all demonic items out there.
My favorite October MC of all time would have to be the Coco Kitty Plushie :] It was the first item I quested when I joined Gaia and I love Coco so much and equip it with almost every avatar I make. Coco also resembles my own cat who I adore heart