The Yu-Gi-Oh! franchise consists of various distinct series with lots of entertaining characters, which makes it have a lot of exciting crossover potential. These series actually have crossed over with each other multiple times, but the biggest crossover was much darker than fans could have anticipated. However, this is what also made it the franchise's best crossover.
Over its decades of history, Yu-Gi-Oh! spawned many series that are largely distinct from each other. While the original anime's direct sequel Yu-Gi-Oh! GX was clearly set in the same world as its predecessor, later series took place in more futuristic settings, with much less connections to their forerunners. The Bonds Beyond Time movie brought together the franchise's first three series with a fun time travel plot, but beyond that, the series were separate. The fourth series, Yu-Gi-Oh! Zexal, had almost no references to the past besides having a hero who is in many way Yugi's opposite. The franchise's fifth anime Yu-Gi-Oh! Arc-V changes that however, acting as essentially a full series-length crossover.
Arc-V features an interdimensional war between dimensions representing the world of Yu-Gi-Oh! GX, 5D's, and Zexal. Versions of characters from each of these shows appear and have a major impact on the plot. It is satisfying to see these characters from different series react to each other and the different dimensions they go to. However, the backdrop of this crossover is brutally dark, with the GX dimension having already invaded the Zexal dimension and indiscriminately turning its people into cards. This may seem somewhat goofy since these people don't actually die, but the way it is depicted is very dark.
The Zexal dimension is a bleak wartorn disaster area after the GX dimension finishes attacking it. Many of the GX soldiers are so brainwashed that they enjoy the carnage they wreak, with one of them even saying that they enjoy hunting the people of the Zexal dimension like dogs. While it is great to see iconic characters like Kite appear in this dimension fighting against their oppressors, it is also depressing to see the once vibrant and happy Heartland City lie in ruins. And seeing iconic GX characters like Aster Phoenix be a part of this brainwashed army is similarly sad (although he is eventually able to realize the error of his ways).
Thus much of the crossover in Arc-V is a mixture of awesome fanservice and bleak drama. That being said, it is still very satisfying to see the usually rarely connected worlds of the various Yu-Gi-Oh! series clash like this. While none of the previous series protagonists appear in this crossover, getting to see a lot more of popular side characters like Jack Atlas is still very entertaining and should satisfy any fans of the previous animes. So while Yu-Gi-Oh!'s biggest crossover Arc-V may be incredibly dark, fans of the franchise should still check it out to see more of their favorite characters in action.