Over the course of Dragon Ball (Z, GT, etc...) Goku has had to fight and defeat a monumental amount of high-powered beings. Fairly recently, he ascended to the level of a God in order to fight mano-a-mano with the God of Destruction, Beerus.
While certainly cool, it doesn't come close to how instrumental and amazing as Goku's fight on planet Namek. In war of the Gods, when Goku fights against Beerus, it is out of necessity, yes, but it is just another in a long chain of imminent threats to the Earth. On Namek, Goku is forced to duke it out not for himself but for the Namekians. It is the first time that the fate of an entire species was in his hands.
Over the course of the fight, it's not like Goku has the upper hand the entire time. Actually, he's pretty much getting his ass kicked for a big part of it.
The fight went on for 19 episodes straight. Let's break that down. At roughly 22 minutes per episode, that means 418 minutes total, or roughly 7 hours of fighting. alotting for the other narratives still happening on screen during that time, it would still be a 6 hr+ endeavor. It may not be the longest fight in all of anime, but it's certainly up there.
This fight enraptured and summarized Saturday mornings for childhoods all over the world. It was the most intense action sequence of all time for (probably) thousands of people.
Really, though, this is what made the fight the most iconic one of all of Dragon Ball. This moment, where Goku has just witnessed his life-long friend Krillin be LITERALLY EXPLODED TO DEATH by Frieza.
It is this moment where Goku has passed byond a mental threshold, becoming so enraged with Frieza and so vengeful over Krillin's death, that he transcends his physical limitations and ascends to nigh-mythical level of Super Saiyan for the first time.
Until this point, there'd only been a little bit of exposition regarding the Super Saiyan form, all from Vegeta, as he has invested considerable time and energy purusing the form for himself. Goku ascends to this unprecendented level of power so as to combat Frieza in his final form, to avenge Krillin, and to save the Namekians.
It was the first time that Goku was pushed to his absolute breaking point, forcing him to shatter his limits and rise to a cataclysmic challenge. For this reason alone it could be considered his finest fight. In all subsequent fights, he pushed further and further, but none had the gravitas of his Frieza fight, and none ever will.