I've gone taken a gander at some maps of NW Faerun and I am not sure exactly which terrain would be useful. My players and I are doing session zero in a couple days and my ranger player has asked me what terrain would be most useful. Despite having looked at the map, I do not know which terrain will be most beneficial as I have not yet read the whole adventure. I figured I would ask someone who has already run it, or has detailed knowledge of the area from other sword coast campaigns.
Few "bad" choices, no obvious choices
Player choice is heavily embraced in this story. The players will be faced with numerous ways to complete their objectives, which could require them to travel to many different places in Faerun.
There is a more-or-less required portion in a mountainous arctic environment, and a more-or-less required portion in a desert environment, but other than that characters could choose to go to any number of environments to complete their objectives. And DM choice (or random chance) could compel them to travel nearly anywhere.
The only choice I would particularly advise against is the Underdark. Few adventure options head in that direction (although the Stone Giant adventure could be reskinned that way).
My honest suggestion would be for your Ranger's player to select a type of terrain they are most interested in interacting with, and let that guide the group's decision on where to travel and which story threads to pursue. If you'd want to know the terrain my particular group interacted with the most (on foot), it would probably be coastal. But again, there's a lot of variety in what your group may want or need to do.