Lightning
I'm kind of getting tired of this canon rehash. Not only Bulma has a radar, there's a second radar he took from her father, so he could go ahead and take some balls while the duo go about getting the easier dragon balls. I mean, sure, Goku gets in some fights and get stronger, but he would get even stronger by using his time training with Raiden or with Kame. And besides, until RR Army appears, all fights are pretty much unchallenging to Goku.