[cellurdur]

Originally Posted by Belegorn

I can't say who exactly is the most powerful Elf in Rivendell. Certainly there were few left for;

"here in Rivendell there live still some of his chief foes: the Elven-wise, lords of the Eldar from beyond the furthest seas. They do not fear the Ringwraiths, for those who have dwelt in the Blessed Realm live at once in both worlds, and against both the Seen and the Unseen they have great power." [FotR, Bk: 2; ch. 1, p. 269]

Gandalf expounds on the qualities of some of these lords dwelling in Rivendell to Frodo. There were Elves from Aman of great power dwelling there. Who the greatest of them is I cannot guess. Glorfindel points out to Aragorn and the Hobbits;

"There are few, even in Rivendell that can ride openly against the Nine; but such as there were, Elrond sent out north, west, and south." [bk:1, ch. 12, p. 256]

Glorfindel even before his death was an elf of great power and a Balrog slayer. Considering Gandalf the grey also died killing a Balrog should indicate his power. After his death, his powers were greatly enhanced and he was closer to a Maiar than an elf.

"For long years he remained in Valinor in reunion with the Eldar who had not rebelled, and in companionship of the Maiar. To these he had now become almost an equal, for though he was an incarnate (to whom a bodily form not made or chosen was necessary) his spiritual power had been greatly enhanced by his self sacrifice."