When Barry Allen showed up in Starling City in season 2, it was an indication of much bigger things to come. Originally planned to have a backdoor pilot on Arrow, they decided instead to give him his own proper pilot, and haven't looked back since. That's led to some great crossover moments between the two, with the most signifiant being the big two-part team-up Flash vs Arrow/The Brave and the Bold. It was the latter of the two that was considered the Arrow episode, and it gave us the awesome moment of The Flash and Arrow teaming up to take down the bad-guy, in this case Captain Boomerang. Seeing the two heroes work together was pretty awesome, and it was all capped by the Rocky III-esque ending.