If you shoot landscapes then a polariser would be very useful. Skylight or UV filters are commonly used to protect your lens from scratches, but it is very much a personal decision whether or not to use one. Many do, others, myself included, don't bother and use a lens hood as protection and am very careful when handling the camera.
ND Grads are handy, but not essential for landscapes. You can achieve the same results in postprocessing. There are so many to choose from. Not only do they come in different strengths from 1/3 to 3 stops they also come in two flavours, hard and soft, which refers to the transition between dark and transparency. Hard are normally used for straight horizons, like seascapes and soft where there are mountains or whatever making for an irregular horizon. I've got a few, but don't use them any more. I take 2 exposures and merge them in Photoshop.
Full ND filters can be useful if you like shooting water and getting that blurred effect.