So, given the immense power levels in the Dragonball universe, especially with Goku’s various transformations, it’s a bit tricky to find a form that Luffy could definitely beat. But if we consider Goku in his earliest stages, sau, from the beginning of Dragon Ball, things get interesting.
Early on, in his journey, young Goku is incredibly strong for his age, but he doesn’t yet have the insane power levels he achieves later. During this period, Luffy’s combat skills, speed and Devil Fruit abilities might give him the upper hand. Luffy’s Gear Second and Gear Third techniques, combined with his Haki, could potentially outmatch Goku before he masters advanced forms like Super Saiyan.
However, as Goku grows and develops his abilities – especially by the time he reaches the Dragon Ball Z and Super Arcs – he quickly surpasses Luffy in raw power and speed. Goku’s Super Saiyan forms and beyond, put him in a league of his own, making it extremely challenging for Luffy to compete.
In essence, while Luffy might hold his own against a very early version of Goku, the sheer power escalation in Dragon Ball makes it unlikely for him to win against Goku’s later forms. So, it’s all about context and timing. Early Goku? Luffy stands a chance. But for Super Saiyan Goku and beyond? Not so much.