I shoot landscape mainly. CPL is hardly ever used, because there's not much reason to use it. I only use it when the reflection in the water gets really annoying. Any landscape photographer will be able to tell you that they dont use CPL for the sky (unless you want the half blue effect), esp for UWA. Not to mention they are thick, which will cause vignetting when stack with 1 other filter.
GND is a must, but not the first filter to get. Most of the time HDR and/or photoshop's GND will get what you want. Only in a few cases is a real GND actually needed. Since you are new, I suggest you fall into the HDR hole first then learn to climb out of it. I feel the process matters.
That said, a 9 stop or 10 stop ND should be your first filter for smooth clouds/water.