Rule of Law. Every law applicant reads it, so give it a go but be aware this will only make you seem at most as average. To impress, you need to do more.
Brandeis and Warren, The Right to Privacy, is a pretty good one if you want to seem really eager. Then you learn what developed from that in the US and can compare with the situation we have had and have presently the UK. You tie up understanding of tort, public law, international comparative law and ECHR into one exercise. But I am only talking about privacy because it interests me. Find an area of law you like and really enjoy reading about, and read about it! Your true passion for that area, along with critical understanding, will shine through at interview. This is better than discrete bundles of law you try to remember without any real point of reference -- which is invariably the case with introductory texts for applicants.