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Trevenant fits the quintessential archetype I pictured when someone said grass/ghost before grass/ghost existed.
Dhelmise has the most interesting spin on the typing, in my opinion. I was really intrigued to read its Pokedex entry and notice the typing when I first caught one (somehow was one of the few new Pokemon I didn't look at prior to playing.
Decidueye looks cool, but the ghost typing seems a little like a superfluous add-on at the end. But overall, I think Decidueye makes it work. Ghostly sniper makes sense I guess.
I think Gourgeist looks like Marzipan from Homestar Runner. Pumpkaboo is cute though.
Overall, my favorite is Phantump.