In short, you ask someone who knows you well and can speak positively about your skills, strengths, and accomplishments from a clinical or academic experience.
Most of your letters should come from within the speciality you are applying to, who know you well.
While it is more important to have a LOR from someone who knows you well, it can sometimes help if the person writing the LOR is well-known within the speciality or sub-speciality as attendings know one another from across the country, so having a reference from a well-respected colleague will make you stand out.
Networking is important in the States, so your USCE and LOR can boost your application.