The second Eternals trailer finally revealed why the group didn't help in the battle against Thanos. Their Celestial overlords forbid them from interfering in humanity's affairs. However, the Infinity Saga was a universal conflict. It wasn't just Earth that suffered the consequences of Thanos' snap, so they should've been allowed to help.
If the Eternals had indeed fought against Thanos, the Infinity War might've lasted only a few days. The group is absurdly powerful, outranking almost every other member of the current MCU. They all have a basic set of abilities that make them near-immortal while also having individual powers. While they all would've proved crucial to secure the victory, some would've been more useful than others because of their unique abilities, battling prowess, or a combination of both.
Sprite
Sprite is the youngest-looking of the Eternals, an "old soul" in the body of a 12-year-old girl. Aside from having the strength, durability, and stamina of all other Eternals, Sprite can also project lifelike illusions and sustain them for prolonged periods.
The ability is nothing to look down on, but it is the least impressive out of the group, at least in battle. Sprite can always use her illusions to confuse her enemies and manipulate their perceptions. However, they're still illusions, and there are limits to them.
Makkari
As the first deaf superhero in the MCU, Makkari is on her way to becoming an icon. She possesses super speed and other related abilities. Mikkari's comic book counterpart can create cyclones by running in circles and can run close to light speed for extended periods.
In the battle against Thanos, Mikkari could've taken the Gauntlet from him and run away with it. However, she would have few opportunities to steal it, especially if it were fully functional. In the final battle of Avengers: Endgame, she could've dealt with several opponents at once, but she would've played a mostly supporting role.
Ajak
Comics fans know Ajak as the male leader of the Eternals. In the movie, the character is female but retains her spot as the team's wise and spiritual guide and mentor. She has the unique ability to communicate with Celestials and also possesses healing powers.
In battle, Ajak would come in handy to heal fallen allies. Acting as a bonafide war doctor of sorts, she would fight to keep enemies away from her healing station, but her primary role would be that of a healer. The extent of her abilities remains unclear, so it's unknown if she could've saved Tony Stark, for example.
Phastos
Phastos is the Eternals' resident creator and inventor. He has a place of honor among the Eternals in the comic books, acting as the team's weapons designer and technology developer and equipping them with most of their gear.
Because of his Eternals abilities, Phastos would be the team's equivalent of a soldier, carrying weapons into war and doing his best during battle. However, Phastos' main contributions would happen before the conflict, when he equipped the team with his devices.
Kingo
Film Kingo has several crucial differences from his comic book counterpart. In the film, he can fire energy bolts from his hands, an ability that most Eternals display in the comics. He also seems to lack his counterpart's ability as a samurai and master swordsman.
Kingo has some of the most "generic" abilities in the Eternals, but he's still a massive threat on the battlefield. His bolts could kill several opponents at once. In fact, he takes down a Deviant with just a couple of fingers in the trailer, proving that his attacks are powerful indeed.
Gilgamesh
In the comics, Gilgamesh also goes by "Forgotten One." Throughout the years, people confused him for numerous mythical heroes, including Gilgamesh and Hercules. His immense physical strength makes him the strongest of all the Eternals. However, it doesn't surpass Thanos', at least not in the comics.
Gilgamesh would be a behemoth in battle. He would tear through Thanos' defenses with ease, crushing his enemies without shedding a drop of sweat. He might not be able to take on the Mad Titan, but Gilgamesh's force would be enough to defeat some of Thanos' most fearsome creatures, like the Leviathans.
Thena
Thena's powers seem to have suffered the most drastic change from print to film. In the comics, Thena has a wide range of abilities and a keen intellect. She carries a bow that releases "cold energy" arrows and a spear that surrounds enemies in a ring of fire. In the movie, she can create multiple weapons out of cosmic energy.
As a gifted combatant, Thena would kill many of Thanos' soldiers with her array of cosmic weapons. Her power's description suggests she could also create cannons, machine guns, and other kinds of massive weaponry, Green Lantern-style, meaning she could take down a considerable part of Thanos' army all by herself.
Druig
Unlike some of the other Eternals, Druig doesn't quite love humans. In fact, he enjoys torturing them in the comics and even spends some time as a KGB agent. His unique ability allows him to control the minds of others, a gift that makes him one of the most dangerous members of the Eternals.
Druig's power would allow him to control Thanos' army and turn it against him. Perhaps he wouldn't be able to enter the minds of the First Order, but he could command the soldiers. It's unclear how many people he can control at once, but perhaps he could even manipulate the entire army.
Ikaris
At first sight, Ikaris might seem like just another of the many versions of Superman. They both fly, have enhanced strength, and shoot laser beams from their eyes. Ikaris has more abilities that differentiate him from the Man of Steel in the comics, but his film counterpart lacks them.
As the MCU's Superman, Ikaris instantly becomes one of the most valuable heroes. Like Captain Marvel, he can make all the difference with his powers, taking down Thanos' ship and even engaging him in hand-to-hand combat. There are limits to his abilities, and he probably wouldn't stand a chance against a Gauntlet-wearing Thanos, but he could give the Mad Titan a decent fight.
Sersi
The consensus is that Sersi is the most powerful of all the Eternals thanks to her unique ability. She can manipulate and rearrange the molecular structure of matter, although the movie seems to limit her power to alter inanimate objects only. In the comics, she can change the atomic structures of all matter, including living organisms.
Even with the limitations, Sersi's powers are ridiculously convenient. She might not be strong enough to alter the Infinity Stones' integrity, but she could manipulate the gauntlet, preventing it from harnessing the stones' power. In battle, she could destroy the enemies' armors and weapons, leaving them completely vulnerable. Understandably, the MCU avoided using her in the Infinity Saga because her powers are so vast that she would've ended the conflict before it even began.