I think Kid Buu is the strongest non-fused character followed by Mystic Gohan and that Mystic Gohan is stronger than ssj3 Gokou. But that's just me. Course the only real thing I have to base that off of is Gokou saying somebody stronger than him would appear and he was talking about Gotenks and Gohan is stronger than Gotenks and Gotenks is suppose to be stronger than Gokou.
Wasn't it made clear the Goku was wanting to hold back because he wanted the "boys" to save the world since he wasn't going to be around anymore? I remember that there was a difference between the Japanese dialogue and the English dialogue at that one part. It occurred after Goku fought with Mystic Buu but before he fought with Kid Buu.
After reading the dialogue in the Japanese version, I got the impression that Goku "threw" the fight with Majin Buu and Mystic Buu.
That confuses me because he fused with Vegeta earlier and Goku thought that that fusion was permanent.
My main point is, Goku would have found a way to win no matter what. Goku has the unique ability to overcome those who are stronger around him. That is his gift and ability.
If Gohan became "possessed" and became evil, Goku would have had trouble fighting his son and would have his ass handed to him at first. Goku would eventually come around and figured out a way to defeat his son...its just the way Goku's character is. However, if neither Goku nor Gohan "became" evil all of a sudden, there would never be a fight between them.
Also, if you compare Goku's strongestto Gohan's strongest, there is not contest. At the end of GT, Goku has 9 balls!!! That's a lot of balls!