Great interpersonal skills is the number one answer. Make sure that you're comfortable having conversations in-person and over the phone with people. Confidentiality is another big requirement for any HR job, so be very mindful about what you discuss and with whom.
In terms of technical skills, advanced Excel skills will always set you apart, especially if you're interested in compensation roles. I've also found that being able to present information (like survey data) in creative and engaging ways has wowed my employers. Get comfortable using SmartArt in Microsoft and Canva online.
Interpersonal skills are invaluable to the job. Employers want someone that is not afraid to walk up to them and engage in a conversation with them because that is what the job calls for. This should be a given, but attention to detail when filling out forms is sought after. When I was interviewing for an temporary internship, the job was centered around auditing I-9 forms. Off hand, the interviewers mentioned that it was good that I knew how to fill out the form because it could take four times before it was filled out correctly.
Memorization was something that impressed my new boss. I took a labor law class and learned a lot about what was legal, technically legal, and illegal. The ability to regurgitate everything that I had learned impressed her and continues to help me with my job.
I have really developed interpersonal skills, organization, the ability to notice a problem and come up with a solution to fix it, excel spreadsheet work, and honestly so much more. I learn so much on a daily basis I am constantly growing in my knowledge of the industry and myself.