The Justice League is DC Comics' greatest team. Since their introduction, the League has been battling the most terrible threats in the universe, from Starro the Conqueror to Darkseid. The League has protected the entire multiverse from every danger and has led Earth's heroic community into battle any time they're needed. Few teams out there have the kind of power and experience the League does.
The Justice League has faced all manner of villains, from solo enemies to teams of terrors. The group has such a great mix of powers that there are plenty of villains who should never fight the League solo. Even ones who have battled the heroes alongside other villains should think twice before messing with the Justice League on their own.
10 The Trickster
The Flash has many powerful villains, but he also fights villains who aren't devastatingly powerful. Trickster is one of the latter. There have been two versions of the character - the original James Jesse and Axel Walker. They have the same basic MO, using weaponized tricks to battle the Flash. Both Jesse and Walker have worked with the Rogues, with Walker being the more vicious of the two.
However, regardless of who is wearing the blue and gold costume, Trickster should stay out of the Justice League's way. Any one member of the team can beat him single handedly, so fighting them would be foolish. Trickster isn't in their league.
9 Trident
Trident is a very interesting villain, in that there have often been several at a time, all posing as the same person. The Trident Trio first debuted battling the New Teen Titans, splintering away from H.I.V.E. to start their own criminal empire. Each wore the same costume and wielded a trident that fired ice, fire, and a devastating force blast.
Since the Trio dissolved after they killed one of their members, there have been many Tridents. As far as it goes, Trident is more of a cannon fodder villain than anything else. Trident should give the Justice League a wide berth, as the team just has too much power.
8 The Brain And Monsieur Mallah
The Brain and Monsieur Mallah have been the core of the Brotherhood of Evil for a long time. The two have worked together for ages, with the Brain having done the experiments that transformed Mallah from a regular gorilla to a super-intelligent French gorilla. Since then, the Brain has been reduced to a brain in a life-support system, with Mallah acting as muscle.
The two are known for outlandish schemes, but the Justice League has faced much worse than them. The League is full of heroes who face off against evil scientists with unique schemes. The Brain and Mallah don't bring anything new to the table, so they shouldn't go after the League.
7 Reverse-Flash
DC's most ruthless villains are a terrifying lot, known for going to horrific lengths to succeed. The Reverse-Flash's obsession with destroying Barry Allen has seen him travel through time, murdering and destroying anything that made Allen happy. Reverse-Flash is a superlatively powerful villain, someone who seems like he could handle the Justice League on his own.
However, looks can be deceiving. The League has Flash on the team, as well as superfast heroes like Superman and Wonder Woman. On top of that, after Reverse-Flash attacked him in the Batcave, Batman almost certainly made a plan to deal with the villain. Add in all the other powerful heroes, and Reverse-Flash doesn't stand a chance.
6 Sinestro
DC's villainous icons are multifaceted and dangerous. Sinestro has always been a rather successful person, whether it be in the Green Lantern Corps, battling said Corps, or starting the Sinestro Corps and becoming the most dangerous ring slinger in the universe. Sinestro can bring it when he has to.
As good as Sinestro is, going against the Justice League solo would be foolish. While he's dangerous and cunning, it all comes down to numbers. He wouldn't be able to hold forever out against the team, especially if there was a Green Lantern in the group. However, even without one, the team would win. Sinestro should be smart enough to stay away from the League.
5 Mister Freeze
Some DC villains are very popular, but that doesn't mean they should go up against the Justice League. Mister Freeze is among Batman's most well-known villains, and has challenged The Dark Knight often. Freeze's armor, freeze gun, and intelligence make him a wily and formidable foe.
However, none of that would be enough against the entire Justice League. Batman beats Mister Freeze on a regular basis, so adding in the rest of the League makes this one a figurative bloodbath. Freeze might be able to come up with a plan that would challenge the League, but he'd never be able to put them away.
4 General Zod
Many DC villains have a myriad of powers, making them extremely formidable in battle. General Zod has all of that and Kryptonian might on his side. The general is known for fighting Superman to a standstill, and his military training makes him a foe that shouldn't be trifled with. It would stand to reason that he could beat the Justice League, but he definitely shouldn't try.
Zod certainly has the power level and skill, but he's a Kryptonian. The Justice League knows all the weaknesses of his race. Batman himself has multiple plans for dealing with Superman if he goes evil; he can use them on Zod. Zod should definitely stay out of the team's way.
3 Count Vertigo
Count Vertigo has been battling Green Arrow for years. A solid B-list villain, Vertigo has been a member of the Injustice Society, the Suicide Squad, the Secret Society of Supervillains, and, in recent years, has worked with Checkmate. His powers, once supplied by an implant meant to help him hear but long since internalized, allow him to disorient his enemies and mess with many types of electronics.
Count Vertigo is useful on a team of villains or against one hero, but he's not going to be able to take down the Justice League. The best he could hope for is keeping them off-balance for a time before they struck back and knocked him out. The League is just too much for him to deal with.
2 Bane
Some DC villains shouldn't switch heroes, no matter how successful they are. Bane has handed Batman some devastating defeats, but that doesn't mean he would be a threat to anyone else. He especially wouldn't stand a chance against the Justice League. While his strategic skills would serve him well, he'd be like a dog chasing a car - he wouldn't know what to do if his plan worked.
Venom would never make Bane strong enough to beat the team, and he's definitely not ready for the mix of powers and skills the League brings to bear. Bane is a formidable villain, but he wouldn't be able to effect the League. Hopefully for him, he'd be smart enough to realize that.
1 The Joker
The Joker is Batman's arch-nemesis and has proven to be a villain who can surprise anyone he goes up against. Even heroes with powers have found themselves tested by the Joker, as his unpredictability and knack for hurting people make him difficult to deal with. He knows how to strike when it's least expected, taking advantage of the altruistic nature of the heroes to get a leg up.
Joker has faced off against the League while on teams like the Injustice League and the Secret Society of Supervillains, but solo is another story. The Justice League is just too powerful for him to pull his tricks with. One of them would take Joker down before he knew what hit them.