Nadal of course. But people will vote for Andre becauae there are more Federer and Djokovic fans combined than Nadal fans, and Fedovic fans ain't gonna vote for Nadal because he is the rival of their idols. They don't want to give Nadal extra credit in the GOAT debate.
Nadal faced much stronger opposition (the 2 best hard courters of all time) and still managed to win as many Slams on hard as Agassi. That alone proves who's better on hard.
Additionally, in the only match they played on hard, a 19 years old teenager Nadal beat 35 years old Agassi. That match occured in late 2005, and Agassi was closer to his prime on hard than Nadal (Agassi reached the USO final a few weeks after this encounter, while Nadal did not reach his first Slam SF on hard till 2008).
Not to mention the MOBILITY problem of Agassi, who could barely run in his 30s. He had a genetic problem on his back (spondylolisthesis) that produced his characteristic "duck walk". Nadal is much faster and more complete. For those thinking I'm "making up" this, check the link below:
"Agassi and approximately 7% of the population suffer from a structural defect in the spine called spondylolisthesis. This condition is characterized by a fracture (lysis) that causes the front body of the vertebrae (spondylo) to separate (thesis) from the bone and joints located at the back of the spine."
Many of us who learned to play tennis in the ‘80’s and 90’s were taught to emulate Agassi’s mini-step style of movement.
orthophysio.com