Word that means "First Among Equals"

How about "preeminent"? I think it fits your definition.

The only single words that I can think of that have ever been used to mean that have lost the meaning through the somewhat natural process where the "among equals" part becomes more or less of a joke: prince and its Latin root princeps.

In phrases, first citizen is commonly employed, and the plain Latin for first among equals, primus inter pares, has some history behind it.

I was going to suggest "primus" as Chaos has already done. Alternatively, "principal" could be appropriate in some circumstances - although I imagine this could be confused with the use as "senior teacher within a school" in the US.