I found that personal reflection is quite useful as it is essayish but allows you more flexibility as you do not need to adhere to the structural conventions which are usually associated with an essay. In my opinion it allows you to draw on the best of both the imaginative and expository modes of writing as it is a hybrid approach, and allows you to exhibit a good writing voice. By way of contrast, in a straight imaginative piece, if you were, say, writing from the perspective of a character in one of the texts you are studying, you would need to be able to convincingly imitate how that character talks and thinks, and possibly the style of writing of the author as well. It is a good idea to try and use a form which gives you free rein over how you write, and which feels natural to you. It is also recommended that if you choose a certain form that you read a lot of examples of that form so that you can develop a sense of how to write in it.