You want:
Polarising filter, to darken blue skies and reduce reflections (which makes grass and foliage etc darker and richer). Standard fare for landscapers.
ND grad, to darken skies and balance them with the foreground exposure - stops them blowing out to white. Try a 2-stops hard-cut for starters. Also check out HRD technqiue (high dynamic range) which does the same thing, and rather better IMHO.
10-stops ND. This is the almost black filter that allows you to use very long shutter speeds in daylight, to blur clouds, waves and water, or make anything that moves disappear (cars, people).
Then learn how to use them properly, and always use a lens hood.