The Demonic Pitchfork is THE BEST monthly collectible because nothing says awesome like a sharp, black, three-pronged piece of metal. It's the best basic evil weapon and it's classy to boot. You really can't beat it.
Favorite of all time would have to be the Wonderland MC for March '08
I just love Alice and Wonderland and seeing it in for the first time made me squeal with girly delight(I actually scared my mom with my squealing XD ) needless to say I scraped every gaia coin I could to get it(I even had all my RL friends sign up to gaia to get more coins) so yea. I have it and I think I can finally die in peace ;3