Two elven lords, who at the time of the events of "The Lord of the Rings" are among the oldest, wisest and most powerful elves in Middle-earth.
To be fair, I would add that of all the elven people at the time of the events of "The Lord of the Rings" the most powerful is still Galadriel, who in wisdom, experience, magical powers surpasses both Elrond and Thranduil. But she's an elf woman, and we're talking about male elves here, so we'll continue.