If there is one series that Masashi Kishimoto's Naruto has been compared to more than anything else, it's Akira Toriyama's Dragon Ball. This is probably due to how similar the two series are relative to other manga. Throughout both series' publications, fanart depicting crossovers between the two, as well as references to each other have been made. The biggest reference to Dragon Ball that Naruto has is hidden in plain sight.
In the ninja world of Naruto, there exists nine enormous monsters, each with a different number of tails ranging from one to nine. These are aptly called the Tailed Beasts and they are arguably the most powerful creatures in the series. It's with the Four Tailed Beast, an angry, red gorilla, that Naruto refers heavily to Dragon Ball's protagonist, Goku.
Even casual fans of Dragon Ball are aware of Son Goku's Saiyan ape transformation. This is a characteristic unique to Saiyans. After absorbing enough light reflected from the moon, a Saiyan will lose the ability to reason and transform into a giant, building-sized ape, and will go on a destructive rampage. The Four Tails being a giant ape isn't the only thing that makes it a reference to Dragon Ball; it's the beast's name that really drives the point home. The beast is referred to as Son Goku, the same exact name as DB's main character.
The reference doesn't stop there, either. In Dragon Ball, Goku was trained by Master Roshi, an old hermit skilled in martial arts. Naruto changes this master-pupil relationship up a little bit. Son Goku the Four Tailed Beast is sealed inside Roushi, a ninja from the Hidden Stone Village. Whereas Roshi is Goku's master in the sense that he trained him, Roushi is Son Goku's master in the sense that he controls the beast and its immense power. To add just a little bit of extra flavor to this reference, Roushi left his village to better understand the Tailed Beast, making him a hermit just like DB's Roshi in his own right.
This is undeniably the biggest reference to Dragon Ball in Naruto. Sure, it's not that far-fetched that two series would incorporate a giant ape into their story, but when the ape and its bearer are named after the master and student from one of the world's top-selling manga series, it's obvious that it is a reference. Naruto hides this Dragon Ball reference in plain sight, guaranteeing that fans of both series can and will enjoy it.