I don't use graduated neutral density filters for my landscape photography. I do, however, use solid neutral density filters quite often to help control exposure times. As I don't use grads I have no use for a system filter holder, so I prefer to use high quality glass screw-in filters. These are quick to use, robust and provide a good seal around the lens when working in wet environments (no having to remove the filter from the holder to wipe the water off both sides!).
Over the years I have tried many brands of ND filter but for the last few years I have stuck with Heliopan, which I consider to be the best option for landscape photography. When buying filters you should consider these points in the following order of importance:
Image quality - there's no point buying an expensive and well-designed lens only to put a poor quality piece of glass in front of it! Heliopan filters use very high quality Schott glass and I have never noticed any significant degradation in image quality when using them.
Durability - Filters are there to be used and should be able to stand up to rigorous use in the field. This includes wiping salt water spray from the filter without scratching it! They should also be durable enough to withstand occasionally being placed unprotected in a coat pocket without scratching. This is where Heliopan filters really excel compared to other brands that I have used. Their coatings are very tough, which means the filters last a long time without scratching. This is a very important consideration because when shooting into the light even tiny scratches will increase the chances of flare ruining your image. Although I normally take care to clean my filters with cotton tissues and microfibre cloths, they have at times been cleaned with paper towels, shirt sleeves and cotton rags with no ill effects. I know from experience that I could not do that with other brands.
Ease of use - You need to be able to quickly attach and detach the filter from the lens. The filter thread needs to be both robust and well-machined so that it screws on and off with ease. Heliopan filters are not the best in this regard, but with practice and care they are acceptable. However, I always carry a filter wrench just in case!
Colour casts - obviously you will want your filters to be as neutral as possible, but many are not! A weak colour cast is easily compensated for in post production (as long as you shoot in RAW), but strong colour casts can mask the natural colour of the scene completely, making it impossible to recover some subtle colours and tonal graduation. The stronger the filter the stronger the colour cast tends to be. The Heliopan 3.0 (10-stop) ND filter exhibits a warm colour cast which is reasonably easy to remove in post and can sometimes enhance a shot.
Price - Heliopan ND filters are priced similarly to those of other manufacturers, but cost a great deal less than some "premium" ND filters.