Not everyone gets to be Thanos.
A good villain is a necessity for a superhero story, but it seems that fans believe that 'Eternals' were not really lucky in getting an outstanding adversary, because the Deviant leader Kro has been anointed as "the most forgettable villain of the MCU" on Reddit.
According to the 'Eternals' mythology, Kro came to Earth thousands of years ago, with Eternals themselves hunting him and his species down just because they were Deviants. However, right before the emergence of a new Celestial from inside the Earth, Tiamut, Kro and his legion had been freed. The villain managed to kill Ajak, the leader of Eternals, and gain her power; every time he killed an Eternal after that, he would snatch their powers and evolve, growing stronger and more dangerous.
But Kro turned out to garner little fan love; and so did Eternals, with the movie receiving mixed reviews. While many people would defend the protagonists, few fans could argue that Kro was impressive enough to praise him as a memorable character.
Some fans think that the Deviant leader was a "missed opportunity".
"I thought he could have made an awesome villain in a sequel, where he wants to destroy the Eternals on a more personal level after having that existential crisis post-killing Gilgamesh and realizes that they are all Arishem's puppets, and maybe he tries to become more than just a feral Deviant as a reflection of the Eternals becoming more attached to humanity. Instead, he shows up for a forced final battle and then dies without much fanfare. Even as someone who liked Eternals, I think he was horribly misused." – /Nhyand52
Well, to be fair, Redditors are always open for a discussion, so the villain from 'Eternals' is not necessarily the only candidate for this less-than-enviable role of the most forgettable MCU villain. Among other "contestants" are Malekith from 'Thor: The Dark World' and Dreykov, the villain from 'Black Widow'.
Among Marvel's best villains, according to fans, are Thanos (naturally) from the Infinity Saga, as well as Loki, Hela and Gorr the God Butcher from the Thor movies. After 'Doctor Strange in the Multiverse of Madness', many people sided with Wanda Maximoff, although it's not quite clear whether she can be considered an actual villain — after all, she was corrupted by Darkhold and had some kind of redemption in the Doctor Strange sequel.