Fiona Timantti. Her atelier is also kind of a hat museum, as she resides in and takes care of the old fashion store of Maria Pelanne, which opened in 1942. The place still looks as it did then.
She has made stunning pieces for showgirls and celebrities alike, as well as hats for everyday wear (which you have seen me model on some occasions, here for example). You can read a good post about why her pieces are "so expensive" - it applies to all other things made by hand as well.