What are the best ebook topics?

Just browse the overall top level "Top 100" Best Sellers in Kindle Ebooks at Amazon.
https://www.amazon.com/Best-Sellers-Kindle-Store-eBooks/zgbs/digital-text/154606011/ref=zg_bs_nav_kstore_1_kstore

You can click the Top 100 Free to see the top 100 Free. Clicking in subcategories won't help you tell what topics are most popular because they try to show the top 100 in each topic and subtopic.

Definitely looks like different kids of fiction are most popular. Not non-fiction.

But if you only want to know what kind of non-fiction is most popular, click an overall Non-fiction category (like Self-Help).

Day with no name
Alfred the ah soul
Zombie donkeys
Heated and soul
Good sams day off

Ones that the writer thought of by himself or herself.

Exploration and travel, architecture and design