In this arc you are introduced to 2 things that are important to the KHR world.
The Vongola Rings and the fact just how spoileriffic the Openings get with showing Chromein direct connection to Mukuro, in my opinion the Varia Arc is the best arc of the series.
We learned a good bit more about the characters we grew to love during their training for the Ring Battles and the action in this arc was also some of the best during the entire series.
Xanxus was an interesting "villain" with a cool backstory and motivation. His Varia group wasn't any less interesting either.
With Squalo being probably the best of the bunch due to his weird sense of honor that still reflects what Yamamoto lives through very well. It was just kinda dumb how Chromes backstory was literally dropped on us the moment she got into her fight, but she does have a good backstory so I am not too mad.
Belphagor was a good opponent for Gokudera, Lussuria probably the best fight Ryohei ever had. Mammon was a key part later on and obviously the best choice for an opponent that Chrome/Mukuro could face…in general the Varia are just a good foil to Tsuna and his friends which I really liked. I was also very happy to see the Varia return later. Much Later in fact. 10 years later, and we have a segway into the next arc!