This is a tricky question, and one we actually can’t answer for you!
Each level has its pros and cons and which level you prefer to teach entirely depends on you.
Some teacher prefer the lower levels, because they worry about their own knowledge of English (grammer in particular!) and not being able to explain or teach more advanced structures.
Other teachers prefer to teach the higher levels because these learners are more capable of holding a more natural conversation. These learners already have the foundations of English and are in your class to learn more advanced aspects of the language.
The same conundrum can be said for teaching Young Learners versus teens versus adults. Each age group presents their own strengths and weaknesses. Which age group you prefer to teach depends on your own strengths and weaknesses, and your preferences.
Read more: Mixed Ability EFL Classes