My all time favorite September Monthly Collectible would be Gothic Butterfly, simply because it's a purple item, and purple is one of my favorite colors. I also thought the wings and other equips were very pretty and elegant.
I like them all as much as i love gaia.
i like them mostly because its coming to winter months and they remind me of beautiful season called fall.
Mostly because the end of the year has the best celebrations like X-mas, Halloween, etc.