Unfortunately there is no one answer to this question for several good reasons:
1. The topic should be about something you like because you’re going to spend a lot of time on it. Different people, different topics.
2. It should be a topic that is included in the IB studies curriculum because that topic will be familiar to those who will guide you through the process and, later, grade the essay.
3. It should relate to your future because the knowledge you’ll learn from the process will help you later in life and will be seen by your adviser as an appropriate topic for you.
4. It should be broad enough that you can write 4,000 words about it and specific enough that there could be more written if you wanted to broaden the scope of your essay.
Of these reasons, the first is the most important because if you like what you are writing about, that liking will show through in your writing and, not only will the whole process be easier, your emotional input will be clear to whomever is grading your essay and just might improve your grade. So it’s not so much about what topic is easiest, it’s about which is easiest and most enjoyable for you personally. So what’s your favorite topic? Your favorite subject? Your favorite anything that falls within the scope of the IB curriculum?
Hope this helps.