Raditz, Goku's biggest challenge?

[SparkyPantsMcGee]

First impressions mean a lot. If he's attached to his family and friends there is a good chance he shows remorse. It's not totally fool proof but Raditz most likely took a gamble off an assumption.

Yeah, but that's all it was. A gamble. He also told Goku to kill 100 Earthlings for an apology, showing that he really doesn't understand.

Actually that's a good point, but not in the way you're thinking. That kind of demand was a perfect way to gauge how different Goku was from a normal Saiyan. Most Saiyans would have shrugged that off as no big deal. Goku on the otherhand, outright refused and answered by attacking Raditz. On top of that, Raditz saw Goku's reactions to Krillin getting attacked and his son getting kidnapped. It was instantly clear to Raditz that, by Saiyan standards, Goku had gone soft. He saw his opportunity when Goku grabbed his tail and played on Goku's soft heart. Sure it was dirty and low, and even a little risky, but it worked. I don't think Saiyans are above graveling if it means they gain the advantage and win.

[Saiga]

Giving Raditz waaaaaaaay too much credit there. It's definitely more about Goku's naivety than Raditz' cunning - Goku didn't even need to be begged to spare Piccolo or Vegeta.