Both of the titles you provided are grammatically correct, but they convey slightly different meanings.
The title "The subject I've enjoyed studying" is in the past tense. It implies that the subject you enjoyed studying is no longer being studied, or that you have completed your study of that subject.
On the other hand, "The subject I enjoy studying" is in the present tense. It implies that you are currently studying the subject and that you find it enjoyable.
The choice of which title to use would depend on the focus of your essay. If the essay is about a subject that you've already completed and enjoyed, then "The subject I've enjoyed studying" would be more appropriate. If the essay is about a subject that you are currently studying and enjoying, then "The subject I enjoy studying" would be more appropriate.