WARNING: This article contains SPOILERS for Avengers: Infinity War
Fans can debate which Avenger is deadliest in the action scenes of Infinity War, but there's no question Thor is now the strongest. That may not make much of a difference given the power of Thanos and the Infinity Stones, but after Thor: Ragnarok showed Mjolnir wasn't the source of the hero's might, Infinity War has taken things even further.
It may be hard to find the boundaries and guidelines needed to define a "strongest" hero, when every MCU superhuman can live through pretty much anything (other than warping reality itself to remove them from existence). Yet Thor's mission seems to be one that only he could survive, coming out the other side having proven to possess more power, more gifts, and more potential killing power than any of his teammates.
How the Thor movie series may tweak his godly power remains to be seen. But in the Avengers story currently being told, there is no doubt the god of thunder is top dog. At least, until Carol Danvers comes into the picture...
Thor is Physically Stronger Than Any Other Avenger
Finding the line between strength, durability, and near-invulnerability is also a challenge, so to keep it simple, let's focus only on a hero's ability to survive lethal force and conditions, and the amount of energy they can output. The biggest leap made by Infinity War comes when the Asgardian refugee ship (piloted by Thor director Kenneth Branagh) is blown to pieces by Thanos. It's unclear how much time passes before the Guardians arrive, but it's more than long enough for Thanos and his ship to depart. Although the time may not matter: even left in the ice cold, airless vacuum of space, Thor is just fine when he splats onto the Guardians' windshield.
So add "no need for oxygen" to Thor's superhuman gifts. Before he wakes, Gamora also notes that Thor's physical makeup - his muscles at least - is also unique. Gamora has seen hulking warriors before (and was raised by one) so we hope the composition of Thor's body is what she's referring to, and not ignoring Drax and Peter Quill's own ripped physique for the sake of a gag. After all, Thor's body has already proven strong enough to survive the Hulk's most powerful attack with hardly a scratch in Ragnarok. In Infinity War, he has strength enough to activate a solar forge.
Oh, and face the full heat of a star, melting metal while giving Thor little more than a sunburn.
Infinity War Shows Thor Really is 'A God'
The idea of Asgardians being "worshipped as gods" by less advanced humans was introduced in the first Thor movie, and doubled down on when Jane Foster claimed Asgardian "magic" was simply highly advanced science. That may be changing, judging by Infinity War, as Loki makes the case that it isn't a matter of perspective - he and Thor ARE gods compared to most other beings.
As the God of Mischief, Loki wields power over perception and the fabric of reality. As Thor learned in Ragnarok, it was never his hammer that gave him his powers. He truly is the God of Thunder, imbued with the bodily power to impact and be fueled by weather, lightning, 'ambient energy,' whatever you want to call it. It's that kind of power Thanos desires, but which Loki reminds him will never be his. That even if the Infinity Stones grant him godhood, it is THEIR power he uses. He "will never be a god" as they are in their atoms and essence.
Once Thor claims Stormbreaker - "the weapon of a king" stronger than any previously wielded in Asgard or elsewhere - any conditions on his power are erased. He no longer needs to be worthy, he simply IS. Whatever powers fueled Bifrost are now his to command personally, if he wishes. Were there any doubts, he's also the only one able to plunge an axe directly into the chest of Thanos, leading the villain to suggest if he had aimed for his head, Marvel's greatest villain would have been dropped on the spot.
Thor is The Most Powerful Avenger... Next To Captain Marvel
Assuming that the Power Stone is a concentrated form of the power that permeates the universe, it's fair to say that Thor is at least partly fueled by a similar cosmic "force." That makes him the strongest Avenger alone.. but not for long. Because once Captain Marvel arrives on the scene, she will wield raw power and energy unlike anything either Thor OR Thanos thought possible.
That's thanks to the Kree genetics she's infused with in becoming her superhero alter ego. As a human/alien hybrid, the Carol Danvers of the comics has been shown to not only have superhuman strength, durability, and survivability in space, but the ability to manipulate energy and power. Not just create power, or channel it through a godly gift - but essentially deflect it. She's used that power to reach explosive capabilities close to a nuclear bomb - but may not actually have an upper limit.
So yes, Infinity War proves Thor is truly a god among heroes... But that's still not enough. Thank goodness Captain Marvel has gotten the call.