The right topic is the one you can relate too; the topic you enjoy.

Now, if you don't mind a bit of shameless promotion, I have something that might be helpful. Everyday I post a breakdown of the performance of the different tags on Steemit. This may help you find topics you enjoy that are also "worth" posting in.
Here is today's:
https://steemit.com/steemit/@calimeatwagon/daily-steemit-tag-rankings-1-100-9-22-2017