Which witch is the titular Witch’s Ghost in Scooby Doo and the Witch’s Ghost?

In this instance, the title is deliberately ambiguous (Like, which witch is the witch?).

By the end of the film (and tie-in storybook) the ghost seen during the twist-ending is called out as being the real witch's ghost and the witch's ghost seen at the start is outed as being fake. It follows, logically, that the title must be referring to both.

Ben had tricked them all — the gang from Mystery Inc. the mayor and his friends, the Hex Girls. And now, his evil plans were about to unfold.
“I will summon Sarah" Ben cried. "Together we will rule the world!"
The book glowed red. The wind whipped fiercely, pushing everyone back. The townspeople flew into the stocks of the Puritan Village, and the Hex Girls were tied to a post.
Thunder boomed. Lightning flashed. Clouds shifted, and suddenly the witch's ghost — the real witch's ghost — swept through the sky.
"Serve me!" Ben commanded.
But the witch's ghost had other ideas! "I serve no one!" She glared at Ben, holding out her arm. A fire blast shot out and caught Ben up in a green glowing ball.