Who was the greatest Elf?

[Yavanna Kementari]

Luthien was the most beautiful..yes.
But if you're talking about, great as in strength, it would be Curufinweor Feanor - Spirit of Fire.

But in the bearing of her son Miriel was consumed in spirit and body; and after his birth she yearned for release from the labour of living. And when she had named him, she said to Finwe: 'Never again shall I bear child; for strength thst would have nourished the life of many has gone forth into Feanor.'

The Silmarillion Chapter 6 OF FEANOR AND THE UNCHAINING OF MELKOR (pg 64-65)
And Miriel died in the Garden Of Lorien.

And their are many other greats. Like Cirdan the Shipwright and Earedil! Just depends on what kind of great.
~Yavanna