The following contains spoilers for the Chainsaw Man manga, Chapter 54, "Go to Enoshima" and onward, by Tatsuki Fujimoto, available now in English through Viz Media.
Denji and Quanxi are both devil hybrids in the Chainsaw Man universe, which is a relatively rare phenomenon that only occurs when devils fuse with a human. Unlike fiends, who at first appear similar, hybrids maintain their human appearance and consciousness, only morphing into a human-devil when triggered, such as when Denji tugs the pull chord in his chest.
When it comes to ranking hybrids, Denji is always considered the strongest, with Quanxi coming in at a close second. However, when it comes to battle experience and innate strength, the devil hunter turned assassin is undoubtedly the strongest of the pair. The only thing holding Quanxi back from the number one spot is her love for her girlfriends.
Quanxi Is Far More Skilled Than Denji
Both introduced and killed in the "International Assassin's" arc, Quanxi's time in the plot is short-lived, but her impact is destructive. Within seconds, and while moving at an inhuman speed, she was able to single-handedly behead many of the puppets that Public Safety was struggling to combat and get closer to Denji than any other assassin. She took down the entire team protecting Denji, including Aki, Angel, Yoshida, Kusakabe and more, establishing herself as an overpowered and unbeatable enemy. Quanxi even defeated Kishibe, Denji's mentor, in hand-to-hand combat -- something Denji couldn't do even with Power's help -- once again proving her superior strength over her fellow hybrid.
Quanxi also has far more experience than Denji. Despite her youthful looks, Quanxi is over 50 years old and is famous for being the first devil hunter, so she definitely one-ups Denji in battle. She's been fighting devils for most of her life, while also being a trained assassin. Her power is so intense that she was able to sneak up on and behead Denji before he even knew she was there, without needing the strength of her hybrid form. Throughout the arc, Denji scraped by due to his inexperience despite being in his powerful hybrid form, but Quanxi dominated the battlefield while staying human the entire time. Denji may be strong as a hybrid, but even without her hybrid strength, Quanxi's assassin lifestyle and combat history trump Denji.
Quanxi Prioritizes Her Girlfriends' Lives
Quanxi is clearly the better hybrid, but there is one thing holding her back from the highest rank. The assassin has four fiend girlfriends who she very much loves, so much so that she would put her own life on the line to ensure their safety. Pingtsi, Long, Cosmo and Tsugihagi are Quanxi's number one priority and the very reason for her undoing. Despite her being able to overpower the battle, as soon as Makima entered, Quanxi could sense the danger and immediately surrendered, begging for her partners' lives to be spared.
She only decided to take this stance to protect her fiend girlfriends, as the heartless Makima wouldn't hesitate to torture or kill them. At this moment of vulnerability, the Public Safety superior beheaded Quanxi and the partners who survived the gory battle. Quanxi is the strongest hybrid, but her admirably protective mindset toward her fiends stopped her from being the stone-cold killer she had the potential to be. On the battlefield, she has no weaknesses; defeat is only an option when her partners are involved.
This isn't to say that Denji lacks humanity -- if he had a partner, he would likely strive to protect them in the same way. While new love interests may on the horizon for the devil hunter, he is currently without that affection, so Chainsaw Man remains the strongest hybrid.