Lewis Carroll doesn't specify what sort of hat the Hatter wears, but John Tenniel depicts him as wearing a top hat (see related link), as does the 1951 Disney movie.
He wears a trilby hat that some people still wear (the style)
A Trilby hat or Deerstalker hat.
kepi hats
Stetson Diamond Jim with a custom hat band.
fedora
what kind of hat did laurel and hardy wear?
chef hat
chef hat
fashion hat
some red hat
A Watteau hat.
it was a bowler hat.
Greek Fisherman Hat.
Fedora, or a felt hat.
he eares a black at
flatbill
Derby/Bowler Hat