So uh, I can now claim to have watched all of the Yu-Gi-Oh! series(the original and the 4 spinoffs) as of last week because I finally finished 5Ds.
The spinoffs are not half bad actually and you can watch any of them without first needing to watch any of the others-though if you watch GX/5Ds/ZeXal you'll get some of the subtle references made to them in Arc V. But it's not required.
It is advisable that you watch the subs though, the dubs, really don't do them justice at all. And in the case of GX and 5Ds, you miss a lot-GX stopped dubbing at the penultimate episode of the 3rd season,meaning the season finale for s3 and all of s4 went undubbed. And 5Ds cut out an 11 episode chunk in the middle of season 2 and the last like, 20+ episodes of season 2.
Not to mention the utter hack job done to the stories(like changing 'death' with 'kidnapped' or 'disappeared' even when it makes literally no sense to do so(it's 4Kids though, so, yeah) and altering or removing significant plot things.
Arc V is on going though, with new episodes every Sunday(with subs usually going up by Wednesday/Thursday).
I mean, putting aside the whole, card game aspect for a moment, the plot lines are actually kinda fun.
GX- Following the adventures of the 14/15 year old protagonist Judai Yuki through what's essentially 3 years of boarding school(each season covers roughly one school year, with seasons 3 and 4 being one year), including the awakening of ancient evil(as in evil from the beginning of creation simply known as the Light of Destruction), transfer students and his past coming back to haunt him in a horrible way. And the tone takes a serious dip into dark by season 3(arguably one of the darker series because of this). Also includes an interesting twist on light and dark and which is good and which is evil.
5Ds- Ancient gods of good and evil have revived themselves and 18 year old Yusei Fudo finds himself one of 5(later 6) individuals marked as the reincarnation of Signers, warriors from the past able to summon the Crimson Dragon(hence 5Ds, 5 Dragons), a being sealed away within the marks. On top of dealing with that is the whole stigma of the fact that he's from Satellite, essentially the slums of the city, and bearing a criminal marker, both things that are looked down upon by the 'Tops' of the city.
The second season delves into a group of individuals from the future who seem intent on destroying Neo Domino City and Team 5Ds' efforts to stop this. Can't say much else without spoiling the second season.
This one, GX and Duel Monsters all canonically exist within the same universe/timeline, falling on after another).
ZeXal- 13 year old Yuma Tsukumo befriends the amnesiac other worldly spirit Astral, that only he can see, and helps him to try and recover his scattered memories, which are in the form of Numbers cards, so he can remember his mission. At the same time, forces both of the human world(season 1) and the Barian world(season 2) seek these numbers too. Some for the power, others so they can destroy that world/dimension and the Astral world.
Arc V(currently on going, at episode 96 as of this post)- 14 year old Yuya Sakaki just wants to follow in the footsteps of his missing father and make people smile with his Entertainment dueling. Unfortunately, there's an inter-dimensional war heading their way and a mystery surrounding both his Pendulum Summoning and his counter parts(as well as the counter parts for his childhood friend Yuzu) in the other 3 dimensions, who all look a lot like him, to deal with first. Massive tonal shift around episode 33 when it gets dark fast.
There are revealed to be 4 dimensions, 3 of which are basically au versions of the worlds of the previous three series(and named after the primary summoning methods introduced/used in each); Fusion(DM/GX), Synchro(5Ds) and Xyz(Zexal). Standard is the 4th and where Yuya is from, it uses all kinds of summoning.
Each season seems like it will be touching on and travelling to each dimension, starting in Standard to set everything up, then Synchro(where we are now and just wrapping up), next up is Xyz(we know this from spoilers/leaks) and presumably Fusion last.