Kid Buu was the strongest.
Do you remember DBZ at all? At the part where Super Buu (basically 3 forms, just depends on who he has absorbed at the time) turns into Kid Buu after Goku and Vegeta take Piccolo, Gohan, Goten, Trunks and Mr. Buu (fat Buu) out of Buu's body, Buu freaks out and says how Goku and Vegeta basically fucked up and he turned into Kid Buu, his normal form.
You'll have to watch it again, Buu wasn't happy about having to turn back into his normal form because even he realizes how crazy he is as Kid Buu.
And at the end when Goku won with the spirit bomb, he was actually really lucky because Buu threw it back at him. Even with the power of everybody from earth (all the Z fighters included) wasn't enough, they had to wish Goku's power level back to normal.
Technically, Buu would've won that shit because Goku was tired, had no energy left from fighting and got lucky in the end. Kid Buu was actually the strongest in the universe up until that point, see Resurrection F about how Frieza was terrified of only Beerus and Kid Buu, the two strongest in that universe by far.
Yes, I literally finished watching DBZ again like 3-4 months ago so I remember all this shit.