If you love landscape photography then a graduated nd filter is a must-have. It’s basically a filter that has a dark & a clear section where the dark section reduces the harsh light that may be coming from the sun or sky. While you can do this in post via HDR, using this type of filter makes your photos look more natural (I actually don’t like too much post-processing, specially HDR).
These (filter) come in two types, the circular which is mounted on the lens just like how you would mount a circular polarizer and the square or rectangular which is mounted on an adaptor. I prefer the square type because it allows you to move the filter up or down therefore giving you the option to tame the light source without sacrificing your composition. The disadvantage though of having an adaptor is that it extends far and causes a vignetting effect.