No man is invincible.

Even the best of the best run into the occasional fist or shin, and they wake up with people thinking they're not that good or that they never were.

That's one of the main rules of the game, and it happens to champions too. Every champion in the UFC will lose at some point.

These are the folks most likely to take titles away.

Flyweight: Ian McCall

McCall is a monster, and when the 125-pound division didn't exist in the UFC. most felt he was the best in the world. He didn't do much to dispel that the first time he fought Demetrious Johnson, who is now the champion and ended that meeting on the bottom of this.

Unfortunately, a bunch of stuff happened and McCall had to fight Johnson a second time. It didn't go as well, and his decision loss pushed him to the back of the line.

The moustachioed Uncle Creepy has gone through some personal issues over the past year, but he's a nightmare for Johnson when he's clearheaded and on his game. The third time could be a charm if he earns another crack.

Bantamweight: Dominick Cruz and Renan Barao

Wait, there are two champions at 135 pounds? But who's the true best? And who's the worst matchup for each guy? Will we ever know?

Here's the thing: Cruz has been out forever, and Barao has been defending the interim title in his stead. Hence the two champions. And they're the worst matchup for each other.

Cruz, with his footwork, evasiveness and wrestling, is a different animal than Barao has ever seen. Barao, with his lethal Muay Thai and patented un-takedownable Nova Uniao approach to the game, could give Cruz the type of fight he's never had.

All we can hope is that Cruz gets right and comes back sharp. The unification bout between he and Barao is far and away the most appealing fight available at bantamweight.

Women's Bantamweight: Cat Zingano

Alexis Davis was getting alot of play as the next possible threat to Ronda Rousey, but her inconsistency has long been an issue, and an underwhelming victory over Rosi Sexton quieted those pushing her up the ladder.

Enter Cat Zingano.

Zingano had earned her chance with a violent stoppage of Miesha Tate, one that would have come with a chance to coach TUF as well had a knee injury not sidelined her until further notice.

When she gets that chance, she could be the one to end Rousey's impressive streak of armbar wins.

Zingano is an excellent wrestler, an excellent grappler and has a finisher's instinct to match any in the sport. When she sees a chance to stop her opponent, her opponent gets stopped.

It may be a while before they meet, but, when they do, Zingano could be the one to finally topple Rousey.

Featherweight: Frankie Edgar

Jose Aldo is quickly climbing the pound-for-pound list in MMA, and the more exposure he gets, the more people are coming to realize just what a special talent he is. His greatest exposure came as a headliner against Frankie Edgar on Super Bowl Weekend this year, and it's Edgar who still has the best chance of toppling the champion.

The man who'll beat Aldo is a guy who can out-quick the champion, a guy who sits atop the division as the pinnacle of speed meeting power at 145 pounds.

Edgar is faster, has good footwork and can wrestle when he decides to. He did well the first time but didn't do enough to win. If he gets another chance, he'll look to ride the aforementioned tools to success.

Lightweight: Gilbert Melendez

There's a pretty solid argument to be made that Melendez actually already did beat the champ when they fought in April, but make no mistake that he's still the man to do it now.

And he's probably not far away from a rematch with Benson Henderson, giving how well he acquitted himself the first time.

His stand-up matches Henderson's well enough, their wrestling is a wash, and where the champion has an edge in jiu-jitsu, you would give the same edge to Melendez in ground-and-pound and killer instinct.

If they meet again, Melendez is the guy to become the new champion at 155.

Welterweight: Johny Hendricks

Let's be realistic here: No one stands as a greater threat to Georges St-Pierre than anyone else these days. Every guy is just another guy that the champion can either take down and ride to a decision or batter from a distance with jabs and kicks to the same end.

For that reason, oddly, every guy is the biggest threat to St-Pierre. With no one name standing above the rest, every name is a viable option.

Hendricks is next on the list, so he's the biggest threat at the moment.

Make no mistake, he'll have a hard go of things if he's going to chase GSP around throwing power lefts in an effort to score a KO, and his wrestling is among the most overrated in the division as well. Essentially, circumstance makes him the biggest threat more than what he has to offer in the cage.

Still, as Matt Serra proved, it only takes one shot to change the shape of the welterweight division in a big way. It could be Hendricks' turn in November.

Middleweight: Anderson Silva

How odd is it to suggest that the biggest threat to the middleweight champion is Anderson Silva? Or to not suggest there's someone out there coming to take his title?

Well, Chris Weidman is the champion, and he earned it the hard way, stopping the greatest fighter in history in front of the whole world when most thought him getting trounced was a formality.

It wasn't.

That said, until he beats a Silva more focused on fighting than shimmying around and making faces, until he beats a Silva who at least has his hands up, it's hard not to think the longtime champ isn't the biggest immediate threat to the Weidman title reign.

We'll all find out at UFC 168.

Light Heavyweight: Daniel Cormier

Cormier has stated that, despite signing on to fight Roy Nelson in a heavyweight fight at UFC 166, he wants Jon Jones for the 205-pound title and he wants him soon.

It's a wise move for the former Olympian, because if he can't beat Jon Jones, there isn't a man alive that can make 205 pounds. who can.

Jones is long and rangy and an incredible wrestler for MMA. The guy who beats him is going to have to be a better wrestler who isn't afraid to wade in with punches and through punches in order to make things ugly.

Greco-Roman guys are generally better suited to do that, and that's where Cormier excels. If he doesn't mind eating some shots to be successful, successful is what he'll be.

That's far easier said than done, however, as Jones has been nothing short of invincible in his MMA career. It's hard to claim anyone as a threat to him at this point.

Heavyweight: Fabricio Werdum

While Junior dos Santos will surely get people excited after the sustained thrashing he took from the champion not a year ago, it's hard to peg him as the biggest threat to the Cain Velasquez era (this one, at least).

Other guys like Josh Barnett and Alistair Overeem are curious options, but are a few fights away at a minimum.

Enter Fabricio Werdum, the one style the world has yet to see matched with Velasquez and the one that may well give him the most trouble.

On his feet, Werdum is incredibly dangerous, a true practitioner of using eight limbs to maximize his output. And on the ground? Well, on the ground he's racked up six ADCC medals and submitted Fedor Emelianenko and Minotauro Nogueira within his past four MMA wins.

A guy with submission skill like that isn't going to be afraid of the inevitable ground fight with Velasquez, and his stand-up is a wash with the champion. At worst.

This is the guy to challenge the heavyweight champion in a way the world has yet to see.