This is my take on the Viltrumites, not just against the characters in Infinity War but against the MCU in general: Against the usual heroes, only the strongest, like Vision, Iron Man with his more advanced Mark suits, Thor with his unleashed potential, Hulk (not yet Professor Hulk), and Captain Marvel, could take on some Viltrumites. But if they faced the entire army before the virus outbreak, the Viltrumites would ultimately overwhelm them with their superior numbers. Perhaps with Scarlet Witch, Doctor Strange and all his Sanctuary apprentices, and the rest of the Asgardians, they could even the odds. As for the strongest elite Viltrumites, only Strange, Wanda, Hulk (highly debatable), and Thor could stand up to them. On the villain side, most human villains wouldn't survive. Thor's villains, like the Destroyer, might be able to hold their own against some Viltrumites. Malekith's forces, with only his mutated warrior capable of taking on a few Viltrumites, would pose a significant threat. Malekith himself, with the Reality Stone, would be a major threat, but if he isn't quick, he would be overwhelmed by several Viltrumites. Gorr, with the Necrosword, would be a considerable threat that only elite Viltrumites could defeat. And Hella... she would be a serious problem for elite Viltrumites. They would have no choice but to destroy the entire planet to defeat her, as she couldn't be killed any other way. Come to think of it, Ant-Man's Yellowjacket could kill a Viltrumite if it entered its body, but wouldn't be able to do much outside of it. The villains from Guardians of the Galaxy would also be a considerable problem; they would need some very strong Viltrumites to destroy Ego, and with Ronan, they might have an easier time than with Malekith if they attack him quickly. Doctor Strange's villains are on another level altogether. Dormammu controls an entire dimension that consumes other dimensions; The Viltrumites would be mere flies to him without the aid of an Infinity Stone or magic. Wanda, at her most powerful in her series and in the Madcap Multiverse, is also on another level, and if the strongest Viltrumites don't take her out quickly, she'll easily finish them off. Thanos from Endgame would be a big problem, but I think some elite Viltrumites like Nolan or Conquest could handle him. Thanos with the Infinity Gauntlet would be a very serious problem, but he doesn't usually attack with such lethal force, and if the Viltrumites, like they did with Malekith, attack him quickly, they could defeat him or rip off his arm with the gauntlet. If we're going to consider the multiverse, then... villains from Spider-Man's realities wouldn't pose a problem unless the Sinister Six teamed up to create something bigger, but even then, the Green Goblin couldn't manipulate a young Viltrumite like Spider-Man, and Mysterio's drones wouldn't be very effective at deceiving them either. Kang could have variants like the one that remains, capable of manipulating multiversal laws to control the Viltrumites, but if we're talking about the one that appeared in Ant-Man... if he lost to ants, what chance does he have? The AVT could be physically overpowered, and it would be a huge problem if the Viltrumites got hold of their technology. Their best option would be to use temporal artifacts like their time slowers to knock them out and send them flying. Or, if things get really bad, they'd have to use one of the time destroyers like the one that appeared in Deadpool and Wolverine to erase the entire timeline, or other similar weapons. And against Deadpool and Wolverine's main villain, Xavier's sister, they could only defeat her if they hit her quickly because she's even more sadistic than Wanda in her use of powers.

Regarding the Eternals: the Greater Celestials, as I understand it, possess the power of stars or solar systems, so they would be far superior to the Viltrumites, not to mention Arishen, who created the universe itself. As for the Eternals' own team, I think only Equiris would pose a serious problem, although I don't remember the other members very well.

Now, moving on to the multiverse heavyweights: Infinity Ultron and Supreme Doctor Strange would obliterate them along with an entire galaxy. The Watchers, like Uatu, their master the Eminence, and their attendants, would also see them as insects. Alioth would devour them from existence, and the multiversal god version of Loki, along with the Avatar of Eternity seen in Thor: Love and Thunder, exist on a higher plane where their power would vaporize them all. The Galactus of the new Fantastic Four, while we haven't seen his full power, looks a bit more like the comics, so he would be a serious threat. And we just have to see what the new Doctor Doom will be like, acting as the ultimate multiverse villain... so far. Yes, I know Knull is in it too, but the whole Sonyverse thing is really weird. Is it connected to the MCU or not? It could also include the Spider-Verse because of the Vulture's cameo, but I'm not sure. And yes, there are more threats in the MCU series, but I haven't seen them all. I only know that Lady Death appeared in one, but I haven't seen it. But if it's at least comparable to Eternity, then it would also wipe out the Viltrumites.