What is the best selling book that RL Stine ever made?

The best-selling book by R.L. Stine is "Goosebumps #1: Welcome to Dead House". This book was the first in the wildly popular Goosebumps series, which has sold over 400 million copies worldwide.

While Stine has written many other successful books, "Welcome to Dead House" was a major breakthrough for him, launching the Goosebumps phenomenon and solidifying his position as a master of children's horror.