When the Fourth Great Shinobi War broke out at the end of Naruto Shippuden, Madara and Obito Uchiha (and later Kaguya Otsutsuki) were the main foes to be faced. That said, it was another person entirely who proved indispensable in his services to the enemy side, and revived Madara in the first place. That man was Kabuto Yakushi, until only recently Orochimaru's loyal aide. When Orochimaru seemingly died at Sasuke's hands, Kabuto injected himself with Orochimaru's DNA and eventually gained many of the same frightening powers his master possessed, as well as those of many of his servants, like the Sound Ninja 4. Kabuto was instrumental in the war due to his activating the Impure Resurrection, reviving many dead shinobi including Madara to face down the Shinobi Alliance.
Kabuto eventually faced down Sasuke and a revived Itachi, who had managed to free himself from Kabuto's control. During their battle, Kabuto claimed to have surpassed Orochimaru, adding further proof to his words by mastering Sage Mode, something Orochimaru's couldn't withstand. Orochimaru has since been revived, and after coming to terms with the error of his ways, Kabuto also became an occasional ally to the Leaf Village like his former master. In Boruto, he lives a humble life running an orphanage. With both Orochimaru and Kabuto alive and arguably at the peak of their prowess, it's a fair question to ask if Kabuto truly has surpassed Orochimaru in strength. It's unlikely that they will ever trade blows, but some assumptions can be made.
Kabuto vs Orochimaru
Kabuto's claim to have surpassed Orochimaru is not complete hyperbole. Orochimaru's body was too weak to handle the Sage Mode transformation, and the fact that Kabuto was able to fight Sasuke and Itachi simultaneously puts him well above Orochimaru in raw power in this state. After absorbing Orochimaru's DNA, Kabuto also inherits most of the former's natural abilties, such as the Power of the White Snake, and the ability to conjure snakes en masse. The only aspect of Orochimaru's power that Kabuto is not confirmed to have inherited is the former's near immortality and fast-acting regeneration. He has not been seen using either of these abilities, so it's up in the air as to whether these are exclusive to Orochimaru.
Kabuto has a leg up on Orochimaru in one more important regard, and that is his mastery of medical ninjutsu. His encyclopedic knowledge of the human body is a major reason for Orochimaru's trust in him, and Tsunade, perhaps the greatest medical ninja in history, openly admitted he was more proficient in medical ninjutsu than she was at his age. All of these factors seem to make it a slam dunk that Kabuto has surpassed his former master.
There are some important caveats, however. Orochimaru is still one of the most experienced and dangerous ninja alive, and Kabuto did not inherit all the countless jutsu he has mastered. The difficulty of killing Orochimaru, especially now that he is in much better health than earlier in the series, would make Kabuto's power advantage a bit less overwhelming. There's also the fact that Orochimaru has had far more time and experience with his abilities than Kabuto, making him potentially more proficient in their use.
On paper, Kabuto seems to have a leg up on Orochimaru, but this hypothetical matchup is an extremely complex one, and any battle between the two would be reasonably close. A solid resolution as to who would win doesn't seem to be in the cards, as neither Kabuto or Orochimaru seem to bear hostility toward one another. Kabuto's now humble lifestyle and his lack of appearances since early in Boruto when he took in the clones of Shin Uchiha also seem to indicate that he isn't interested in using his power to fight anymore.