Heading into UFC 173 on May 24, there were five fighters who could all lay claim to the distinction of being recognized as MMA’s pound-for-pound champion. Those fighters were Jon Jones, Jose Aldo, Renan Barao, Demetrious Johnson and Ronda Rousey.

Heading into his headlining title fight against T.J. Dillashaw, Barao was in a position to strengthen his claim to the throne and further separate himself from the rest of the pack. However, things did not go as planned for Barao. The massive underdog, Dillashaw, soundly beat the champion in every aspect of the fight, ultimately stopping Barao in the fifth and final round. Dillashaw handed the Brazilian his first loss since 2005 and the first TKO loss of his career.

With Barao out of the conversation now, there are four fighters left for whom arguments could be made. Jones, Aldo, Johnson and Rousey all make compelling cases for that top pound-for-pound spot, but as we saw at UFC 173 no one is unbeatable. On any given night, any one of these four could fall. Of these four, who is the best and who is the most likely to lose next? That is what we will discuss today.

Ronda Rousey, UFC Women’s Bantamweight Champion, 9-0

Without question, Rousey is the most controversial name on this list. But when you look at her body of work thus far in the sport, it is simply impressive. Not only was Rousey the first female champion in UFC history, she was the first woman to sign with the organization. That in itself is an accomplishment, considering UFC President Dana White said it would never happen on multiple occasions prior to 2012.

In her nine professional bouts, Rousey has yet to taste defeat. She has finished all nine of her opponents, and eight of those finishes came in the first round. With elite Olympic-level judo and a quickly evolving striking game, Rousey has separated herself from every other woman in the world as the best. Being better than every other fighter of your gender has to put you into the discussion.

With that said, though, Rousey is not deserving of the top spot on the pound-for-pound list. That’s not through any fault of her own. Women’s MMA just isn’t at a point where the level of competition is truly elite. Although there are a handful of very talented women in the sport, there just isn’t a depth of talent. That lack of overall talent skews the perception of Rousey. Even though she is the least likely of everyone on this list to lose next, she is also the furthest away from that top spot.

Demetrious Johnson, UFC Flyweight Champion, 19-2-1

Johnson was an elite fighter at bantamweight and earned a shot at then-champion Dominick Cruz, but he went to another level when the UFC started its own flyweight division. Johnson looks like a fighter possessed now that he is fighting at his true weight.

After fighting to a draw against Ian McCall in the semifinals for the UFC’s tournament to decide the inaugural flyweight champion, Johnson has been on a roll in his last five fights. Johnson’s first win came over McCall in the rematch to send a fighter to fight for the flyweight belt. Then, Johnson picked up a split decision victory over Joseph Benavidez to become the UFC’s first-ever flyweight champion. Johnson has since defended his title three times, and each performance has been more impressive than the last—he won a unanimous decision over John Dodson, submitted John Moraga in the fifth round and knocked out Benavidez in the first round of his most recent title defense.

Johnson is one of the most complete fighters in all of MMA. He has a great wrestling base, strong submission skills and a varied striking attack. Where Johnson really sets himself apart, though, is in his footwork. His ability to stay out of danger while creating angles for attacks is amazing. But, much like Rousey, Johnson’s ranking suffers because of the lack of depth in the flyweight division. It is extremely unlikely that we will see him lose anytime soon, though, so as others fall off, he will continue to ascend in the rankings.

Jose Aldo, UFC Featherweight Champion, 24-1

Currently riding a 17-fight winning streak, Aldo is right at the top of the list in any pound-for-pound rankings discussion. Aldo has not lost at fight since November 2005, and that was in a fight contested at lightweight. Since being named the first-ever UFC featherweight champion, Aldo has defended his title six straight times, but he has looked less than superhuman on several occasions.

Although Aldo is unquestionably a great fighter, there are some concerns with him, the biggest being his cardio in the championship rounds. Part of this can be attributed to his pace to open fights, but the rest is a result of seemingly poor weight cuts. Aldo is a monster at featherweight, and considering he is still only 27, those weight cuts are just going to get harder and harder. Eventually, he will need to make the jump to lightweight.

Even with his cardio concerns, you would be hard-pressed to find a fighter in the featherweight division with the skills necessary to take the champion out. That is saying something, too, considering the featherweight division is a talent-laden weight class. When you take the depth of the featherweight division into consideration, along with the list of names Aldo has already beaten, he makes a very strong case to be considered the pound-for-pound kingpin. However, he isn’t there just yet.

Jon Jones, UFC Light Heavyweight Champion, 20-1

Of all the fighters on this list, the person most deserving of the pound-for-pound crown is none other than Jon Jones. Jones has shown himself to be an absolute killer in the cage with 20 career victories and 15 career stoppages. His lone loss came in the form of a disqualification for illegal elbows against Matt Hamill at The Ultimate Fighter: Heavyweights Finale. Prior to the disqualification, Jones was utterly dominating Hamill.

Since beating living legend Mauricio “Shogun” Rua for the title at UFC 128, Jones has defeated a who’s who of MMA elite with wins over Quinton “Rampage” Jackson, Lyoto Machida, Rashad Evans, Vitor Belfort, Chael Sonnen, Alexander Gustafsson and, most recently, Glover Teixeira. In the most talented division in all of MMA, Jones has been a dominant force. The scary thing is, at only 26, he has yet to truly enter the prime of his fighting career.

With elite wrestling, vicious ground-and-pound, an unpredictable striking attack and the longest reach in the UFC, no one has been able to solve the problem that is Jones. Although there is no question he is the best pound-for-pound fighter in the sport today, he is likely the most susceptible of all the fighters on this list when it comes to suffering a loss. That is not a knock on Jones, but rather a fact of life in a light heavyweight division where fighters like Gustafsson and Daniel Cormier are chasing after Jones’ belt. There are just so many talented fighters with finishing ability that no one is truly safe.