I'm guessing it'll depend on what you want to do, if you are going to do long exposures on a full time basis then you'll probably best to invest on a Grad ND filter set, I've see the results from Cokin and Hoya and they seem to be a good brand, albeit rather expensive in my opinion.
If it's for the occasional long exposure you can just get a variable ND filter, they're quite versatile and affordable.
I'm just playing around some long exposures these days and I use a Hoya variable ND that gives me up to 8 stops, I also have a NiSi ND 1000 that has 10 stops, I stack them now and then depending on how much stops I want. I find both brands to be good and doesn't give any colour cast to the images.