I stayed at Claudio's apartment with 2 friend for Carnaval. This location in Leblon is perfect for Carnaval. Several of the blocos that go along the beach start on his street, so it is very easy to get into the action. At the same time, the street is also quiet, so you can escape the noise for some sleep. The Leblon beach at Posto 12 is the least crowded and least touristy, equally beautiful.
The apartment itself is very comfortable. We slept 2 ppl on the bed (could probably fit 3) and 1 person on the massage table. We kept the pull-out couch as a couch to sit on and watch TV (no cable, but you can connect a laptop via HDMI). The backyard has a beautiful garden and is a perfect spot to eat breakfast. The kitchen has everything you need to cook. There is a washing machine and drying rack. The shower is strong and hot.
There is also a doorman that is very friendly and helpful with anything you need. I felt very safe in the neighborhood and apartment.
Lastly, Claudio is very helpful in terms of introducing you to the apartment and neighborhood. He is always available via WhatsApp.