I am an undergraduate student and I'm about to take a senior thesis course. However I'm not quite sure about the topic that I want to choose; I certainly have some overall sense about what I would like to choose (like I know that I prefer it to be geometric, so if I was stuck between say algebraic topology and algebraic number theory, I would have chosen algebraic topology), but I was wondering if it's OK to ask MSE users for suggestions based on my personal preferences?
No. This is not a question that is helpful to anyone except you, right now. It won't even be helpful to you in three years, unlike a mathematical question which you might return to (I know I return to mine).
Moreover, the answer depends greatly on who are your potential supervisors, what they can or want to supervise, as well as what are your specific inclinations and preferences.
Find a supervisor, have them make a suggestion, follow it.