If we compare the Witcher 3 to the Witcher 2 antagonist. Is there actually anything "better" about Eredin than Letho?
Letho and his witcher buddies felt more like a threat althought they were only 4, while the Wild Hunt is a full military unit. Probably because Letho actually achieves something. He kills 2 kings and spreads chaos on the North, while Eredin fails over and over again. He fails to convince Ciri to poison the Aen Elle king. He fails to prevent her escaping out of Tir Na Lia. He fails to capture her during the 6+ years, he chases her. He fails to capture her at Kaer Morhen and Skellige, only to get killed shortly after. His only "victory" was Imlerith killing Vesemir.
Letho is physically stronger. Both gameplay- and storywise. The fight against him in the elven ruins is much more difficult than the fight against Eredin at the end of Witcher 3 and Geralt is defeated by him while we always defeat Eredin in the only fight we have with him.
Letho is more cunning. He outsmarts Geralt while killng Foltest. He outsmarts Iorveth, Phillipa and Sile and all 3 of them are considered to be smart as well.
Eredin on the other hand does nothing worth mentioning.
Furthermore Letho is a omnipresent during the story. While we barely meet him we know most of the time what's going on with him, smartly told via conversations, narrations, flashbacks and short encounters. We only meet Letho 3 times (just like Eredin) but because we actually have northworthy interactions with him and we see Letho having interactions with others (Iorveth, Serrit, Auckes) we get the chance to get a deeper look at him.
Both have a personal connection with Geralt established before the events of the games, but while the history between Letho/Geralt is used as a tool to give the antagonist more depth, the history between Eredin and Geralt doesn't play any role.
Maybe their motives differ? Both have basically the same aims. To ensure the continued existence of their "kind" (Aen Elle in Eredins case and Witchers of the Viper School in Letho's case). I don't see much difference. The motives of both are not very convincing given their actions. I would even say Letho's motives are more credible as he got captured by Nilfgaard and was maybe even forced to work for them.
Eredin is no grey character like Letho. He is a clearly characterized as evil. We are not supposed to sympathize with him. That's fine. But then he has to have other traits.
Vilgefortz for example wasn't a grey character as well. But he still worked for me as the antagonist, because he was threatening. He was at least powerful.
Not even that can be said about Eredin. I don't want him to be physically over the top powerful. That's not how he is supposed to be. I want him to be a threat because of this role as the king of a powerful race, who has access to the military forces of this race. But unfortunately even as the leader of the Red Riders he almost never felt like a real threat. Most of the time he and his Riders attacked simple villages without any defense. The only time they actually ecountered resistence they got defeated. (though the problem in this case were the over the top powers of Ciri)
For me personally Letho is the better antagonist in every possible way by a wide margin.