I have a passion for taking landscapes, nature, travel, and long exposures.
Hello there, thank you for the question! If you had to have just one single filter then probably a circular polarizer would be best. (It would be a toss up for me between that or a graduated ND8 filter. You would need a filter holder and the adapter for the filter holder to use the ND8 though so the circular polarizer has the advantage in terms of simplicity.)
When the conditions are right it really does add that extra pop to your images by darkening the blues of the sky and seemingly saturating the greens of foliage a bit. Oh and there is no such thing as a physical photography filter that will sharpen your images by the way; when you are adding glass/plastic in front of your lens you actually degrade the quality in terms of sharpness a little bit. The photos below will show you the effect of the circular polarizer; the one on top without polarization and the second one at maximum polarization.