I find it’s valuable to speak to references who have worked with the candidate in different roles:
Peers provide fair feedback. They often competed or worked closely together and are often the most impartial checks.
Managers offer the best insight on day-to-day interactions and work quality.
Direct reports tend to be a bit more positive than others. But they provide insight about culture, team building and mentoring.
Close colleagues are the least valuable checks. Friends at work don’t have much direct experience to share save for personality.