Well, there's Trevenant and Gourgeist, who seem to be ghost/grass types pairs, and then there's Aegislash, who's in a three Pokemon evolution. Personally, I believe it's Aegislash.
However, I would like to point out that not all types have regional counterpart, and I believe ghost being one of them. There really isn't a main ghost type across the generations, for example; Duskull as you had mentioned is a ghost type from gen III, but is just as common as Shuppet, another ghost type introduced in gen III, The only difference between them is that Shuppet is more common in Sapphire than Duskall and vice-versa for Ruby. Then there's Sableye.
Back on topic about which is the regional ghost of Trevenant/Gourgeist vs. Aegislash, I would personally believe it's the latter, Aegislash, because it has a full evolution line and seems to be its own Pokemon not in any sort of pairs.
