I think that had Goku had more time to train while alive in his body in SSJ3 he could easily surpass Mystic Gohan with the amount of training that man puts in. I mean Goku acknowledged that he had little experience with the transformation, with his fight with Kid Buu being the first time he used it in a full fight with a living body.
Super Saiyan 3 is such a breakthrough that it leads to extended levels of fatigue, even long after powering down. It has its setbacks. If the user has died and is not encumbered by a living body, this strain is reduced somewhat, and additional energy depletion is minimized (such as in Goku's case during his first fight with Majin Buu). So just imagine how much strain it was on Goku to transform twice against Kid Buu.
With lots of training and stability is found, who knows how much powerful a SSJ3 can be. It's possible Gohan at his best in Mystic could possibly be weaker than a Goku who can fully control the power of that transformation. So really while it can't be argued that Mystic Gohan was the strongest character in the DBZ series (Buu Saga), it's really not much of a fair comparison due to the variables. And I don't know if I'm wrong or not but right now isn't the strongest in order:
Whis -> Bills -> SSJG Goku
