Let's try to make this as simple as possible...

1. Start by identifying topics you are most knowledgeable and passionate about. 
2. From that list, identify which ones have the highest demand.
3. From the shortened list, identify which ones are the easiest to create/maintain.

Knowledge and Passion

You can't teach what you don't know. You won't teach well that which you lack passion.

• You don't have to know everything, or be a world renowned "expert" or "guru" to be knowledgeable enough to teach a course. You just need to be far enough down the road to be able to help guide others who are farther back behind you.
• Which topics do you find yourself drawn to talk about with others?
• Which ones give you a sense of energy or charge when you are talking about them?

Make a list of the topics that meet the criteria above. There can be as many or few as you see fit. 

Demand

From your list, which of the topics are people most interested in (aka are searching for)?

There are tools online to see popular keywords and phrases that people are typing into search engines (like Google and YouTube). You can also use the "auto fill" or suggested searches option to see some of the most common phrases people are searching for. These can be really quick and easy ways to see how much interest there is for specific topics.

From your list, mark the top 5 to 7 topics people are most interested in.

Easy To Create

From your list of 5 to 7 topics, which would be the easiest for you to create? That is the one to start with. 

If they are equal, then choose one that calls to you most.
NOTE: At this point you can't go wrong, you just need to choose one to get started.