During the height of its power, when the Holy Roman Emperor met European kings outside of the empire, did protocol indicate he was a superior or an equal to them?

I Think that when you speak about "Height of Power" you should make it clear if you're speaking about the Hohenstauffens or during Charles V in the early 16th century.

If you're speaking about the Stauffen, than the accepted norm in medieval christian europe was that the Holy Roman Emperor was the highest ranking head of state and only the Pope was his equal (that is, at least until the Investiture Controversy when the pope was made in effect superior even to the HRE)

When Speaking on Charles V and onwards two things you need to remember:

The Protestant Reformation absolutely shredded any possible remnant of Unity in Western Christendom (that is, the Christian part of Europe that wasn't Orthodox).
Both the Reconquista and the Hundred Years War have created in Western Europe the nucleus from which the modern Nation-State has developed, in central europe for comparison, no such process has happened and so the creation of a viable nation-state was delayed to the 19th Century.

So, given all of the above you can definetely say that from the time of Charles V onwards his status as the highest ranking head of state was gone for good, and fellow monarchs in europe such as Henry VIII of England and Francis I of France treated the HRE as an Equal and not as a Superior.