The Flash is one of the core members of the Justice League and he proved the usefulness of his skills over and over again. He has a rich plethora of his own villains but also fought more than one common enemy alongside the Justice League.
His main power is his superspeed which makes him the fastest man alive (residing on Earth, anyway), but the Flash is also clever and resourceful. But even his powers have a limit and if he went over to the Marvel universe to fight its villain, he'd defeat some of them but would lose against others.
10 Could Defeat: Loki
Loki isn't weak, not by a long shot, but he's not a real heavy hitter like other villains on this list, more of a trickster. Plus, there are other Asgardians who are stronger than him, such as Hela or Thor. Loki likes to use tricks and mind games to confuse his opponents, and it's likely he would try to use the same strategy against the Flash.
While the speedster might fall for it at first, he's not only fast but also clever and he would quickly learn from his mistakes and would be more careful in the future, so when would the time come to stop Loki, the Flash wouldn't let Loki's illusions faze him and would stop Loki anyway.
9 Would Lose: Dormammu
Even Doctor Strange, one of the most powerful and magically gifted Marvel superheroes, had a hard time defeating Dormammu. Unlike Doctor Strange, the Flash doesn't have magical powers and if he relied only on his speed in the fight against Dormammu, he wouldn't emerge victorious from the battle.
It's a simple question of numbers in this case. Dormammu has a large army of demon-like entities at his disposal and fighting them would eventually tire the Flash out. Even if the Flash managed to somehow miraculously defeat all of Dormammu's army and come out of it unscathed, he'd still had a hard time coming with an effective strategy of bringing Dormammu down.
8 Could Defeat: Kingpin
Both Daredevil and Spider-Man had a hard time defeating Kingpin, but in a way, the Flash is more powerful than both of these heroes. Daredevil has heightened senses but he doesn't possess super-strength, and Spider-Man is also more agile than strong.
In a physical fight, Kingpin relies on his considerable strength but he doesn't stand a real chance against someone as fast as the Flash. The Flash would see Kingpin's punches coming long before the villain could actually hit the speedster and thanks to his speed, the Flash can haul Kingpin to prison before Fisk realizes what's happening, ending the fight in the process.
7 Would Lose: Juggernaut
Put speed against brutal strength and it's almost guaranteed that the second one will win. That would also be the case for the fight between Juggernaut and the Flash. Juggernaut has been able to successfully take on some of the physically strongest Marvel superheroes, such as the Hulk or Colossus, to name just two.
He's one of the villains who fit the description unstoppable and he can keep on using his considerable strength until he wears his opponent down or just knocks him down entirely. Unless the Flash figures out to get off Juggernaut's helmet, he's going to lose the fight.
6 Could Defeat: Bullseye
Bullseye may not have special powers like so many villains on this list but that doesn't mean he's not dangerous and no superhero should underestimate, at least not if their skin isn't bulletproof. Bullseye is an excellent shooter and he proved over and over again that he can seriously wound people or even take them down entirely with a single shot.
He has no trouble killing people, not like the Flash, who tries to avoid killing his enemies whenever he can. That seemingly puts him at a disadvantage against Bullseye but even if the two fought, the Flash is fast enough to avoid all the bullets and knock Bullseye out before he can hurt him.
5 Would Lose: Annihilus
Annihilus is one of the classic Fantastic Four villains and while he may not be as known as other names on this list, he's nonetheless a force to be reckoned with and would most likely be able to defeat the Flash, if the two fought each other.
Annihilus is one of the most powerful beings who reside in the Negative Force and he uses the Cosmic Control Rod which makes him even stronger and more resilient. He has the cosmic power at his disposal and if that wasn't enough, he can also control the minds of others which gives him a distinct advantage over the Flash.
4 Could Defeat: Crossbones
The Flash isn't bulletproof and he can be hurt by ordinary weapons, but his speed usually stops that from happening - unless he'd decide to take the hit for someone else, an innocent bystander, but even then is the Flash usually fast enough to get both the potential victim and himself into safety, out of the gun's range.
If Crossbones and the Flash fought, the result would be the same as in the fight between the Flash and Bullseye. Crossbones would take a few shots at the Flash but thanks to his speed, the Flash would evade them and stop Crossbones before he could actually hit anyone.
3 Would Lose: Thanos
The Mad Titan Thanos is one of the strongest beings in the universe. He's powerful enough in the Marvel Cinematic Universe, but even more so in the comics, and he's easily able to take down a group of fighters without breaking a sweat.
Unless the Flash would manage to catch Thanos when he was weakened, without his powers and dying, he wouldn't have much chance against Thanos. When Thanos died and the Death brought him back to life, his strength grew and nowadays, it seems like nobody can stop him. Add the Infinity Gauntlet to the mix and Thanos is able to stop the Flash in a flash (pun intended).
2 Could Defeat: Black Cat
Another iconic Spider-Man villain on this list, Felicia Hardy aka the Black Cat is agile, athletic and strong. She can hold her ground in the fight against Spider-Man and others (such as the Kingpin), but she'd probably lose against the Flash who can stop her before Black Cat manages to hit him.
The only potential issue for the Flash would be the Black Cat's power which can bring bad luck to him. But even if she used it against him, and he would stumble and fall or something similar, he's still fast enough to catch up with her and stop her from getting away.
1 Would Lose: Galactus
Known as the Devourer of the Worlds, Galactus is one of the strongest cosmic beings in the whole Marvel universe and it would take more than just an effort of one superhero, even as fast one as the Flash, to stop him. Galactus is weaker when he's hungry but still strong enough to overpower one human.
He can easily move a planet and fight anyone who crosses his way. Yes, he's been defeated before, but to one the fight over Galactus, the Flash would either have to team up with other heroes or find a special strategy on how to bring the giant down.