Who would you consider to be Goku's greatest enemy? I know considering he never battled the same enemy twice, but who gave him the most trouble in your opinion? I vote Freeza.
Frieza, Vegeta, King Piccolo & Chi-Chi
Vegeta is more of a rival than a nemesis.
My first thought was Frieza, but there's definitely a strong case to be made for Dr. Gero. Gero held a death-grudge against Goku ever since he took down the Red Ribbon Army as a kid, and worked tirelessly creating androids for the sole purpose of killing Goku for, what? Twenty? Thirty years? That's some grudge.
Yeah, I think I'd have to go with Gero.