Mystic Gohan is the winner.
"@soldier said:He had trained for ten years. "
"@Newdeath: If this happens right at the end of dbz I think that ssj3 goku would win for sure. "Why, he wasnt that much stronger. "
"Why is this even a battle? Toriyama said himself that the strongest un-fused character in the series was Mystic Gohan. "
" @dbzns said: Goku said that both he and vegeta would be defeated if they fought super buu. Mystic gohan manhandled buu. ss3 goku has an enormous chi drain, and he can't stay in that form for very long. While mystic gohan has no chi drain at all. so in a battle of endurance gohan also wins. "
"I'm with Goku. Gohan didn't train for 7 years and never actually achieved anything beyond ssj2. Mystic Gohan was obviously stronger than a ssj2 and at around ssj3 (give or take a little.) Either way it's close. Now if Gohan could turn ssj1 or 2 while in mystic form, then undoubtedly he is stronger but we don't know that he can. Also being stronger doesn't mean he would beat Goku. Gohan's cockyness led him to an early defeat by Buu and worse, he was asorbed. Goku never made that mistake. Also Gohan never fought Kid buu, only Super Buu who at the time hadn't absorbed Gohan or Piccolo which were his greatest assets. "