For landscapes....... In my opinion & in order of importance/usefulness.
CPL
ND solid filters
ND Graduated filters.
Personally, I have & use all three. Many others don't want to or need to use any filters. Have a good search around & read up about them all & decide if you might want them.
On-a-budget Screw filters - Haida, Hoya....
....Square filters - Hitech, Cokin....
Of course, there are always people selling used too.