By far, the single greatest transformation was Goku going SSJ for the first time. It was a culmination of hurt, anger, pride and vengeance. He didn't even know what he was doing. All he knew was that Frieza killed Krillen in cold blood and the same thing would happen to them all if nothing was done. He snapped. His powers began spiralling out of control. Next thing he knows, he's got unlimited power... and destroys Frieza. 2nd greatest was Gohan going SSJ2 for the first time. It rivals Goku's first very hard.
Well like I said earlier on my post Vegeta when he tunrs Super Saiyan the first time it's all heart, he's just mad that he can't achieve it, and then he starts being emotionel and that's why he's the best Super Saiyan transformation in my mind.
Canon, non-canon, that doesn't matter. What matters is that you follow the material exactly or that you are able to find a means to which certain non-canon material can become canon.
I know it's beyond your understanding. But as a wise man once said, a man that removes things he doesn't like from reality only removes himself from reality. What does that mean? Well if you knew there was two versions of Jesus. One which involved Jesus or Yeshua (his real name) as being an ordinary individual that had a relationship with Mary Madalene and praised that people can find hope and peace through their inner self rather than from the church and the other was the complete opposite of that then you would understand how the world works. But wait what does that have to do with anything? Well take it like this. There are two versions and one is far more commonly used than the other, which do you think people will understand?
By the way the one involving Yeshua being an ordinary man is the canon one.
Anyways, back to the topic.
I only like the second level of Super Saiyan transformation with 11 year old Gohan.
And truth is Super Saiyan 4 shouldn't be associated as a super saiyan form, rather simply as an Oozaru form.
Maybe calling it Legendary Saiyan or Oozaru Saiyan.