I don't think the angelic halo is actually the rarest item. There are definitely way more than 10 active users with angelic halos around. The rarest items would be medals or more recently, rare chance items that are in few supply. for example, i think there are only 1 or 2 of these
and
around, etc. there's always the chance they'll be re-released since they're not MCs but that possibility won't happen until next year. it's weird though because it seems like this (last?) year the bundle rares didn't drop much, probably because everyone and their mothers were buying ashens with their GC.
There's also only one of these
around in all of Gaia. So the halo is definitely not the rarest item on Gaia at the moment. It's the highest valued though, I think.
the alchemy final form backwings are also rare/wanted items, even though technically their supply is limitless.
in terms of quantity though, the halo is actually not the most rare item on Gaia. in terms of perceived value it is the most expensive rare though.