My mayor is wearing the plain black cap because I think it fits with her general look right now. I haven't seen all the hats and accessories, but out of the ones I know of, my favorite is probably the halo.
i really like the headband hair, all the boppers (star one is the best tho), hibiscus hairpin, and any of the flowers. and the QR codes people make of flower crowns are adorable, i really hope in the next game they'll add buyable flower crowns or just generally more hair stuff.
The princess crown, my mayor in my main town and cycle/third town almost always wear them . The mayor in my second town wears the Royal crown which is also a favourite, it goes better with her default outfit.
I completely forgot about all the great Halloween masks! I love Halloween in real life and I can't wait for it in animal crossing love the werewolf mask and the ghost mask. Literally just had a mummy shirt sent to me by a villager and made me realise I'd forgot all about them