I would say communication skills. Developing one's communication skills will go a long way in developing trust within a team. Communication, when focused on listening and verifying what is heard, will enable a manager to understand the needs of the team and the organization. Communicating both good and bad news frequently will help a manager to be proactive in minimizing the "rumor mill" and unfounded gossip common amongst managers that do not communicate on a timely manner.
As a consultant, when I meet with clients and conduct an needs assessment, poor communication is usually the number one problem. Strong communication skills will set the foundation for becoming a successful manager.