I'm going to say this right now: the only season of this show that I consider to be completely canon (meaning that I accept everything that happens in the season) is Total Drama Island. Everything in that season is canon to me. Everything else, in my eyes, is neither not canon at all, or it's canon to a degree.
The reason I have this mindset is because, to me, all the characters have been ruined by this point. Or, at the very least, pretty much every character who has been in more than one season. In Total Drama Island, I liked all the characters. Obviously, there were some I liked more than others, but even my least favorite characters, I had nothing against them and I enjoyed watching them. The only reason they were my least favorite was because I wasn't the biggest fan of the archetype character that they represented. But even then, I still liked them.
But in the later seasons, the characters don't feel like themselves at all. I watch Total Drama Island and think about how the returning characters act in the later seasons, and my brain just cannot make the connection; the characters just feel so...wrong to me. It's seriously like every character on the show has been replaced by a poorly programmed robot. They look like the characters, but as far as I'm concerned, they aren't the character.
I don't like disliking characters; I really don't. That's why I stick to Island, because there, I like to watch all the characters.