Halloween in Argentina, tragically, doesn't really exist. It isn't a calendar holiday and most of the people don't celebrate it. However, some clubs, bars, English language schools and party minded people have prevailed in importing the holiday. That means though there is no trick or treating or costume rush, you can still find a small selection of Halloween themed candy and decorations in grocery and party stores, and on Halloween you will see some people dressed in costumes.
Speaking of costumes, it's important to note that the majority of costumes here are rather modest in comparison to the strip club style outfits usually seen in American clubs. I was blind sided by this fact, and turned to the internet to find something culturally appropriate. I decided on a Day of the Dead girl costume because A. It was cheap (A$20 for face paint) and B. It was Spanish style (kind of considering Mexico is like the farthest Latin American country from here). Anyway, the costume turned out really great and earned me many compliments, including an "awesome" from an American guy and a "genius" from an Argentine girl.
So we moved on, and came across Liv, a bigger club not to far away in Palermo. It's part of a small cluster of clubs located next to and across from each other on the same street, and we chose it because from the people waiting and working outside it seemed the most festive. Inside we found around 80% of people with costumes, black and orange decorations, a life size monster, a face painting station and an overall amazing party. It was full, but not overcrowded; it's a pretty large club with a good sized dance floor, 3 bars and VIP table. There was reggaeton music, and mid-party they threw glow sticks into the crowd, which makes it a perfect choice in my book.