Before we get into the big stuff, it should be noted that with a number of our favorite characters interacting for the first time, there were a ton of memorable little moments throughout the course of this crossover. Diggle's deadpan reaction to Supergirl's display of her powers was perfect,
as was Heatwave's odd chemistry with Kara (talk about an unlikely pairing).
It was also great from both a character and logistical standpoint to see Cisco gift Kara with a device that will allow her to cross between dimensions at will, which should facilitate more crossover events like this.
It was also intriguing to see Professor Stein learn that he had caused a time paradox which created a daughter he had never previously had. Struggling with that knowledge and ultimately deciding to risk damage to the timeline in order to preserve the existence of this new daughter gave Victor Garber a chance to flex his considerable dramatic muscles, and it will be interesting to see what ultimately becomes of this new character.