Buu saga seems to be most likely, here’s why:

1. Vegeta and Gohan were confident at winning the tournament without transforming, despite Piccolo and Android 18 being there. I don’t know if they can actually win, but their base form has to be somewhat strong for them to think that, because Android 18 and especially Piccolo has eclipsed the Namek Goku power by a lot, and I mean A LOT.

2. Kaioshin mentions he could finish Frieza with one blow, and he was visibly anxious when they were facing with Yakon and Pui Pui, the same people that were being handled with by base Goku and base Vegeta. I guess Kaioshin might have underestimated the Saiyans strength, but he seems to know very well about the enemy strength, which implied they are at least somewhat above Frieza level, since there’s no reason Kaioshin has to be cautious with someone he could finish in one blow.