In the age of digital photography where special effects can be achieved through Photoshop, filters are not so much in use. However the following filters are good to have when doing landscape photography.

1. polariser – improves color saturation, cut off reflection, make sky to be more blue. Cut off the light by 2 stops so watch out for the shutter speed in case it get too low.
2. Grey Graduated – Mine are from the affordable Cokin P121 series. It helps to balance the big difference in the exposure required for the bright sky and the foreground. It will change the color of the sky due to the color cast but treat it as special effect.
3. Orange – for the black and white photographer to add contrast between the blue sky and white cloud.