Since there's a lot of discussion surrounding this with the recent news of Broly, I'd like to reflect on Dragon Ball as a series in terms of continuity. So let's cut to the chase. Does Dragon Ball - the multi-media franchise that rivals series like Pokemon and Star Wars in popularity - have a canon?
No.
At least, that's the short answer. There has never been an official source that has ever stated, "This thing is canon, and THIS thing is not canon." Try it; look it up. You'll never hear the words "canon" in any quote in any sort of context. Most quotes "proving" there's a canon such as Toriyama comments on GT DO NOT confirm there is an established canon.
Source: http://www.kanzenshuu.com/translations/dragon-book-gt-toriyama-intro/
Being a side-story doesn't mean "non-canon" it just means it's not part of the main story of the series.
However, there's a lot more to it than that. Generally, things in Dragon Ball focus on different continuities of sorts. A lot of the franchise fringes on what-ifs, alternate timelines, other continuities, etc.
However let's get one thing clear, the "canon" of Dragon Ball is completely fanmade. That isn't to say we can't use the term when describing things, or debating things, but it's still largely a fanmade concept for the series. Primarily because TOEI doesn't want to delegitimize their properties by touting them as "non-canon" when they know they can keep the cash flow going in for series like Dragon Ball GT for example.
Now, moving on from that, IF there was a canon, what would it most likely be?
The first tier of canon:
Well the most canon piece of media out there would be these two manga. First is Jaco the Galactic Patrolman and the second is Dragon Ball. Both of these manga were written and drawn by the creator of the series Akira Toriyama with no collaboration from others.
The second tier of canon:
Battle of Gods and Resurrection F.
Dragon Ball Super (anime), Dragon Ball Super (manga), Dragon Ball Super: Broly.
This is where the waters get muddy. Akira Toriyama wrote all of these, but unlike Jaco and Dragon Ball he actually had outside collaboration. Even Battle of Gods was initially a TOEI concept that he retooled and rewrote. On top of that, Super retells the movies in it's own way as well.
Also, despite this forum's hatred for Toyotaro, there's never been an official distinction or point made to suggest the anime is "more canon" than the manga, it's only assumed as such because the anime has a higher priority.
This establishes that even amongst Toriyama's own writing, there's already multiple continuities and different takes of the same story. Everything outside of this is questionable canon, even amongst Toriyama's other writings.
Here's some stuff that MIGHT be canon:
Bu-bu-bu-but what about Dr. Slump?! Let me ask you this: have you READ Dr. Slump? If you have, you shouldn't be asking this question. Dr. Slump kind of exists in Dragon Ball, but it's more of an easter egg. In continuity it doesn't work. All we know is that there's some form of Dr. Slump's story in the Dragon World, not that the actual Dr. Slump manga is "canon" to Dragon Ball.
Yo! Son Goku and Friends is still good...right? Maybe. Nobody really knows. It's referenced in Battle of Gods, but that reference was removed from the Super anime and manga. So...maybe? I don't see why not.
Dragon Ball Online! That's canon! Even though Toriyama worked on it, we do not know to what extent. If he did, he probably forgot most of the stuff he came up for it, lol. Nothing in Super references DB Online, and all of the characters from it have been adopted by the video games. So maybe elements of it will be canon, but we won't find that out anytime soon.
Things that Toriyama didn't write that people mistakenly believe he did:
Bardock: Father of Goku.
Toriyama did not write this whatsoever despite being a fan favorite movie. He drew Bardock and his friends' designs, and then later referenced the movie in his manga by showcasing a panel with Bardock in it.
Dragon Ball Z Kai.
Despite being closer to the manga than the original anime, this isn't the "true" version of the series. There's still a ton of filler moments and stuff in it that wasn't in the original manga. Toriyama also had no hand in this series whatsoever. This is completely a TOEI product meant to make the original anime more watchable to modern audiences.
So what's non-canon then?
Dragon Ball GT, Dragon Ball Xenoverse, Dragon Ball Heroes, and all the original Dragon Ball movies.
There's a bunch of other shit too but you get the gist of it, lol.
Not even good enough to be non-canon:
Dragon Ball: Evolution.
