While Naruto is known as one of the strongest characters in all of shounen animanga, many fans have begun to downplay his strength following the loss of the Nine-Tailed Beast Kurama in the Boruto: Naruto Next Generations manga. Nevertheless, there is good reason to believe that the Seventh Hokage is still one of the strongest characters in his universe.
Naruto Uzumaki lost Kurama back during his fight against Isshiki Otsutsuki, as the Nine-Tailed Beast sacrificed himself so that his host could use Baryon Mode to fight the Otsutsuki. The Jinchuriki was led to believe that both he and Kurama would pass away. However, only Kurama's life force was expended.
How Naruto is still one of the strongest characters in the series
One must remember that Naruto is one of the very few remnants of the Uzumaki clan and its clan members were known to possess incredibly strong life forces and chakra reserves. Hence, even if Naruto were not to have Kurama's chakra, he would have been strong to begin with. Additionally, he was also the son of the Fourth Hokage Minato Namikaze, meaning that he had an ample chakra reserve right from his birth.
While he lacked chakra control, his ample chakra reserve was showcased to fans in the first episode itself, which saw Naruto create 1000 shadow clones. A similar feat was shown when he was creating the Rasenshuriken, all while learning how to add chakra natures to his jutsu. With the character having grown much older, his chakra reserves must only have become bigger.
As for his different forms, while the character did lose the Tailed Beast Mode and Baryon Mode, he still has access to the Sage Mode. He perfected the technique his master Jiraiya never could, that too, without help from Kurama. In fact, Kurama was an active hindrance because he didn’t allow Pa and Ma toads to help the Jinchuriki gather nature energy while he moved around.
As fans would know, Sage Mode multiplies one's base strength and durability by several multifolds. It was also once revealed that Orochimaru's Curse mark multiplied the host's power by 10 times. Considering that Sage Mode is a much more complex technique, the Seventh Hokage's already huge chakra reserve may get multiplied by over a minimum of 20 times while using the Sage Mode.
If fans remember, he was also able to defeat Kurama by himself while using the Sage Mode. That was the battle that allowed the Jinchuriki to extract chakra from Kurama. His feats against Tailed Beasts seem even more impressive when one remembers that the Jinchuriki tanked the Ten-Tailed Beasts' attack in his base form, all while protecting over 10000 shinobi by sharing Kurama’s chakra.
Additionally, as evident from The Last: Naruto the Movie, the Seventh Hokage is capable of punching a hole through the moon. This was after he tanked an explosion capable of creating a huge explosion, all while being in his base form.
Lastly, in the Versus Momoshiki Arc, the Seventh Hokage was seen fighting Momoshiki Otsutsuki in his base form. He indeed ended up activating his Tailed Beast Chakra Mode, but it was only when the fight seemed difficult in his base form. Therefore, there is good reason to believe that Naruto is still the strongest without Kurama's help.