One of the best things about the Arrowverse is getting to see the heroes from all corners of The CW's DC shows come together. What started simply enough with Arrow has developed to span nearly 10 series, with the crossovers getting bigger each time around.
There have been some smaller crossovers, like when Barry Allen debuted on Arrow or when Felicity Smoak appeared on The Flash. However, the big ones see stories that cover several episodes of multiple shows, with the heroes teaming up to face some of the biggest threats. These are often among the best installments of the entire Arrowverse.
Elseworlds, Part 2 (Arrow) - 9.0
"Elseworlds" was the fifth major crossover event in Arrowverse history and it was arguably the most pivotal towards the future. It introduced important aspects like Gotham City, as well as key characters including Lois Lane. The finale even set the stage for "Crisis on Infinite Earths."
The second part aired as an episode of Arrow and continued the fun "Elseworlds" twist with Barry and Oliver switching bodies. Fans enjoyed seeing their favorite characters in opposite costumes. To try and solve the problem, the heroes headed to Gotham, where they met Kate Kane, aka Batwoman.
Invasion! (Legends Of Tomorrow) - 9.0
The third crossover event, "Invasion!" was also the first to include the Legends of Tomorrow. This worked well due to the history some of the characters, like Ray Palmer and Sara Lance, had with those from Arrow. The storyline centered on aliens coming to Earth.
DC's Legends of Tomorrow played host to the finale of this set of episodes and allowed everyone to get in on the time traveling antics. That included stints in 1951 and 1987. While in the latter year, Martin Stein accidentally created a paradox leading to him having a daughter, showing this had a lasting impact.
Crisis On Earth-X, Part 4 (Legends Of Tomorrow) - 9.0
If audiences thought that "Invasion!" was weird, they hadn't seen anything yet. The scale of these events was widened with "Crisis on Earth-X," which saw the universe's greatest heroes get invaded by people from an alternate world where the Axis powers won World War II.
Seeing the franchise's beloved heroes beat up Nazis, even if they were evil versions of themselves, was a blast. The Legends of Tomorrow finale was well-received for the emotional death of Martin Stein and the touching double wedding of Iris and Barry and Oliver and Felicity.
Legends Of Yesterday (Arrow) - 9.0
The second time that the Arrowverse went with a relatively big crossover was the aptly named "Heroes Join Forces" two-parter. In terms of sheer scale, it's much smaller and really on centers on Arrow and Flash working together to do battle with Vandal Savage.
"Legends of Yesterday" was the second part, seeing the culmination of their fight but it also did something more important. This installment managed to set the stage for the yet-to-debut Legends of Tomorrow. For Arrow itself, this played a part in Oliver and Felicity's issues as he didn't tell her his secret about William.
Legends Of Today (The Flash) - 9.0
The "Heroes Join Forces" crossover began with The Flash's "Legends of Today" episode. It came at a pretty intriguing time since both The Flash and Arrow were dealing with major villain storylines of their own, yet still introduced another big bad.
Vandal Savage arrived in Central City looking to kill Kendra Saunders, which the immortal had done in all of her lifetimes. That forced Barry to stop his attempts to be faster than Zoom and Oliver Queen to take a break from Damien Darhk to handle this situation.
Elseworlds, Part 1 (The Flash) - 9.1
The Flash has often been used as a way to begin these crossovers and that was the case again with "Elseworlds." Thanks to events on Earth-90, reality was rewritten, causing Oliver and Barry to wake up in each other's lives, which their teams refused to believe.
Getting to see Barry and Oliver use the powers they aren't used to in order to escape together was a treat. The same goes for seeing them travel to Earth-38 for a meeting with Supergirl and eventually Clark Kent. It was clear that Stephen Amell and Grant Gustin had a blast in these roles.
Invasion! (Arrow) - 9.1
It could be argued that the "Invasion!" crossover was the franchise's best. It did a fantastic job of blending the character interactions fans adored with the action they hoped for and the wackiness that comes with a storyline about aliens. Arrow had the third installment of this event and it went all-in on the strange.
Oliver, Sara, Ray, Diggle, and Thea were abducted by aliens and went through a shared hallucination of a world where Oliver's parents were alive and he never went on The Queen's Gambit. The heroes ultimately worked together to escape and prepare for a final battle.
Invasion! (The Flash) - 9.1
The Flash again marked the start of a crossover with "Invasion!" It is Barry Allen who first investigated a meteor crash that turned out to be an alien spaceship. The species was known as the Dominators, setting up the antagonists of this event.
Knowing he needed all the help he could get, Barry enlisted the assistance of the Legends of Tomorrow, Team Arrow, Supergirl, and Thea Queen. However, the revelation of Barry's manipulation with time through his "Flashpoint" antics caused a good chunk of the team to no longer trust him, splitting them.
The Brave And The Bold (Arrow) - (9.2)
While it is certainly a ton of fun to see so many heroes come together, it's telling that the highest-rated event is the simplest one. "The Brave and the Bold" was the conclusion of the franchise's first true crossover, "Flash vs. Arrow," which fittingly pit the heroes on somewhat opposing sides.
Barry questioned some of Oliver's harsher tactics as they looked into tracking down Digger Harkness. They overcame the odds but it was interesting to see them not be perfect partners. This episode also saw Diggle propose to Lyla and ended in a friendly duel between Oliver and Barry.
Flash vs. Arrow (The Flash) - 9.3
The first crossover started with The Flash and saw the franchise's two most prominent heroes come together. A metahuman with the ability to make others go into an uncontrollable rage named Roy Bivolo faced Barry and Oliver, using his powers on the speedster.
That made it so Oliver had to do everything in his power to stop Barry. With Oliver not having special powers, he had to use his brains and skills to combat him. Together they do take down Bivolo before heading to Star City for a showdown with Digger Harkness.