While it is fairly obvious that the Flash could run circles around Thanos under normal circumstances as his powers make him virtually untouchable, there wouldn't be much the Scarlet Speedster could do if he was around for Thanos' Infinity Snap as there is no way to outrun the god-like power of cosmic chance.
The Flash would overwhelm the Avenger with his nonstop attacks. The Flash has proven fast enough to create tornadoes and has even time-traveled.
DC's speedsters are some of the most potent combatants ever and Reverse-Flash is faster than all but Barry Allen or Wally West, both of whom would be able to defeat Thanos.
That villain is, of course, Kang the Conqueror, who's set to be the Avengers' next opponent in the upcoming Avengers: The Kang Dynasty. And while Thanos is certainly a hard act to follow, Kang may be a far greater threat than the Mad Titan ever was.
The Asgardian Son of Odin was extremely close to ending Thanos at the end of Avengers: Infinity War. After gaining the powerful weapon Stormbreaker, Thor becomes the wielder of the most powerful weapon in the universe. Stormbreaker is capable of killing Thanos outright, and only Thor is currently worthy of wielding it.
In a direct physical speed matchup, The Flash will definitely trounce Thor.
1 Thanos / Darkseid
Of the numerous villains that each team has faced over the years, few have been as dangerous or iconic as Marvel's Thanos and DC's Darkseid, alien warlords obsessed with obtaining a MacGuffin(s) of unimaginable power that will grant them dominance over an abstract facet of existence.
The Flash Would Run Circles Around Black Panther
Nothing Black Panther could do would help him against the Flash. Any attack could be vibrated through and while T'Challa is much smarter than Barry, Barry knows how to use his speed as a weapon in ways that would give Black Panther a run for his money.
1 No Single Justice Leaguer Has Ever Beaten Darkseid
Then there is Darkseid. While the Flash has been able to at least subdue him with the help of the entire Justice League, Flash wouldn't stand a chance against Darkseid in a solo fight.
As the current Flash in DC Comics, Wally West is one of the fastest men alive, easily able to outpace people like Superman and manage to keep up with people like Barry Allen, but a long time ago Wally admitted that the Reverse Flash is much faster than him.
4 FLASH WOULD BEAT: Loki
Loki constantly uses his magical background to either aid or ail his brother, more often the former. Loki's cunning mind is one of his most powerful assets, but facing off against someone who is able to move faster than he could think would put him at a disadvantage.
In a competition of speed between the Blue Blur and the Scarlet Speedster, it will take more than math and science to really comprehend their overall speed. However, taking all things we know into account, Flash would win a typical race against Sonic the Hedgehog and come out as the faster of the two.
Quicksilver, Marvel's Answer to The Flash
The other is in the Marvel Cinematic Universe, Quicksilver is played by Aaron Taylor-Johnson in Avengers: Age of Ultron (2015). Less popular, but probably because this version of Pietro didn't last long. Regardless, there aren't many things this speedster can't do.
"That's about 2,000 newtons, about 450 lbs of force — so he's hitting him with a force equivalent to 450 lbs. and does it in 1/10th of a second."
But for the results on record, Wally is the clear winner. As we can see from the results here, the common folklore compromise that Superman can beat the Flash—any Flash—if he's allowed to fly in the race doesn't actually seem to bear any truth.
Reverse-Flash: The Reverse-Flashes are the speedster arch-nemesis of each Flash. Rival: The nemesis of Jay Garrick. Unlike most his successors, he doesn't wear a reverse of his Flash's colors, rather he looks like an evil version of Jay Garrick.
9 The Flash
He was so fast that he could manipulate time and travel between dimensions. Furthermore, the velocity at which the DC superhero moves makes his attacks much more powerful. If The Flash is smart enough, he can beat anyone, gods included.
And yes, The Flash has the POWERS to defeat Scarlet Witch:
-- The Flash can use a Strong Punch to Knock her Out cold meaning put her to Sleep?.
The Flash would absolutely destroy Iron Man in a fight, and like Machinehead, the Flash would do so in the funniest way.
However, Ultron, who was fascinated with the Stones, effortlessly killed Thanos by cutting his body in half with a laser beam from the Mind Stone and obtained all the Stones for himself which turned him into a cosmic being.
As per the user, the director of Avengers confirmed that Thanos was most scared of Iron Man. “Not because of his strength, not because of his power, not because of his power, but because of his suit,” he added.
Wanda Mastered Her Powers After Clashing with Thanos
Before Multiverse of Madness, she was strong but not that powerful. That's why the Infinity Stones-powered Thanos could withstand Wanda's attack in Wakanda and then defeat her and destroy Vision.