Answer:
I would say an oligarchy.
Explanation:
If the tyrant were to take over an oligarchy, they could convince a few of the members to side with them and betray the rest. Or, they could convince all of them if possible. It would be easier to convince a small group of people rather than a large group like in a democracy. Also, since oligarchies are kind of in-between monarchies and democracies, it may be easier to convert it back into a monarchy.
This is what I think. It may be very wrong, for which, if it is the case, I apologize in advance.
Also, I'm pretty sure you can pick whichever and write about it since the question makes it seem like it's an opinion essay. I could be wrong, or you probably already know that, but I'm letting you know just in case.
Hope this helps you!