The Greatest of the Eldar

[Manwe Sulimo]

If you want to go for the greatest *suggested* Eldar, you can't go wrong with the Three Kings of the Three Kindreds, Ingwë, Elwë, and Finwë. You could probably add Olwë in there, too. After all, they were the kings of the Kindreds, were the first to see the Light of Telperion and Laurelin, and highest in the favour of Manwë. To go even further, Ingwë was the highest, for

The smallest host and the first to set forth was led by Ingwë, the most high lord of all the Elvish race. He entered ito Valinor and sits at the feet of the Powers, and all Elves revere his name....The Vanyar were his people; they are the Fair Elves, the beloved of Manwë and Varda, and few among Men have spoken with them."

Silmarrilion pp.52-53.
If you are going by the opinion of the Lord of the Valar or the Eldar themselves, it's Ingwë.