[The Saucepan Man]

... being Turgon's sister-son makes you a pretty special person.

Special, maybe. But one of the two greatest Eldar?

I believe that Galadriel and Feanor are both referred to as the greatest of the Noldor (although which one tops the chart is a matter of some debate). So, that puts her up there. And then there's Fingolfin. I think he deserves to be pretty near the top. And his brother, Finarfin (Galadriel's father). And Finwe, father of Feanor, Fingolfin and Finarfin. And then there's Thingol. And Olwe. And Fingon, Turgon and Gil-Galad. And a good many others. To me, Maeglin comes pretty far down the list, when we are talking about the greatest of the Eldar.

But going back to the original question, the quote was:

Thus all seemed well with the fortunes of Maeglin, who had risen to be mighty among the princes of the Noldor, and greatest save one in the most renowned of their lands.

This says that Maeglin was mighty among the princes of the Noldor. Princes must be used pretty loosely here, because Maeglin was no son of a King. And there were others who might be desribed as "princes of the Noldor" in the same sense who I would think of as mightier than him. He was mighty, yes. But not the mightiest, or one of the mightiest.

The second part of the quote does not, I believe, refer back to the "princes of the Noldor", but simply says that he was the greatest save one in Gondolin. And that one, must surely have been Turgon, the ruler of Gondolin. Which puts him above Glorfindel and Tuor, but not above other "Noldor princes" in Beleriand.