By far, it's the ND filter, without it you will have a hard time achieving a shallow depth of field without blowing out your exposure. Circular polarizers are neat too for getting rid of reflections like off of windows or water. But basically get a Fader ND and be happy!
Yes, what confuses me is that there are different ring diameters and most of the brands claim that they are fit for all DSLR lenses. For example, a 77mm Tiffen ND filter is said to fit to a 55mm lens. Similary, a 58mm ND filter by Amazon basics is also said to fit 55mm lens. This confuses me a lot and am not able to make the right decision. I am grateful if someone suggests ND filters for 55mm and 200mm Nikkor lenses. Thanks.