Ebook is the most recommended, I know a lot of people that wrote books and got something out of it. Even if it didn't sell well, they learned a lot while writing said technical topic.
The other two requires a lot of marketing and business development more than programming. The HN posts about side projects make it look like it's as easy as `git push heroku master` and 'post on HN', but it's not. The successful ones are outliers rather than the norm. Don't get me wrong, you'll also learn a lot while writing your own game/SaaS. Finishing a game/SaaS on its own is a big achievement no matter how small, but you did ask for a "Best passive income method".
The other two requires a lot of marketing and business development more than programming. The HN posts about side projects make it look like it's as easy as `git push heroku master` and 'post on HN', but it's not. The successful ones are outliers rather than the norm. Don't get me wrong, you'll also learn a lot while writing your own game/SaaS. Finishing a game/SaaS on its own is a big achievement no matter how small, but you did ask for a "Best passive income method".