I've answered a similar question a few weeks ago and that also links to another answer with sources in the short answers to simple questions thread.

The preview to the answer here is that we shouldn't really think of any medieval realm as "unified" in a modern sense but the HRE was certainly the most powerful centralized realm under the Ottonians & Salians as they could rely on the "Imperial Church System." Things got difficult for central imperial authority from the "Investiture Controversy" with the papacy and internal conflicts with the voting nobility.