Why you may ask? Because I seen the Death Battle between Goku and Superman and I just lost a bet. I say Goku should have won. I got nothing against, Superman, in fact, I like both Goku and Superman, but pit them against each other? I would put my money on Goku. But for those who want details, Let's see what Superman and Goku accomplished.
He was able to be very strong despite his young age during "Dragon Ball." By DBZ, He was able to fight and take down some of the more notorious villains, such as Freiza, Cell, Buu and even the Legendary Super Sayian, Broly. During his fight with Freiza, Goku managed to obtain the status and form of Super Sayian. He added aother level after his bout with Freiza (when everyone on Earth thought his game was over), reached SS3 during the epic battle with Buu and obtained SS4 during "Dragon Ball GT." Yeah, he may be tenacious, unbalanced even, but Goku is one tough cookie and if he don't deserve to be in the next Smash Bros., I don't know who does.
Clark Kent started out as a bit of an outsider, knowing he was not of Earth, but he quickly adapted to life on Earth and call it his home. When he grew up, he moved to Metropolis and worked at the Daily Planet. He may be Clark Kent, but when evil strikes, he quickly becomes the famous Man of Steel, Superman! Villainy, beware, for nothing can bring Superman down. Actually, there has been some villains that came close to defeating him. Doomsday happened to be one of them. But Superman came back and continue to protect his adopted home. He battle many villains like Doomsday, Mongol, even his oh-so infamous nemesis, Lex Luthor. Superman became a hero for those who need it most, to fight the battle we never could.
Both Superman and Goku share the same origin. Superman was sent to Earth by his Kryptonian father, Jor-El, before his real home planet, Krypton, was annihilated by it's own core. Goku was sent to Earth from his real home planet, Vegeta, before it, too, was destroyed, but not by it's own core or a asteroid, like some would believe. Planet Vegeta was destroyed by a tyrant named Freiza for fear the Sayians would eventually rebel and defeat him. Both are super strong and fast and you knocked them down, they get right back up, better, stronger and faster then before.
To me, Goku has the advantage because he's basically the only one who can go Super Sayian and Superman can't do that because he's a Kryptonian, not a Sayian (big difference). I have to give Superman credit, though. He's got some powers Goku don't like laser eyes, super breath, arctic breath and can absorb the reys of Earth's yellow sun.