Ok. Now, I know you're thinking "Man, this guy is insane." but here's why I think it's his worst.[]
1.He gets a heart virus and turns weak.[]
2.Vegeta turns stronger than him twice.[]
3.Goku admits "Soon, Gohan became stronger than me." or something like that.[]
4.Goku gets his butt whooped (sorta) by Cell[]
5.He dies by Semi - Perfect Cell (that one is not all that bad due to why)[]
*I forgot that he lost to Android 19 (that's kinda embarassing)[]