The phrase "busiest room" refers to a place or space where there is a lot of activity or where many people are gathered, making it the most occupied or crowded area. Full definition
Place the crate in a quiet room, or a quiet corner of a somewhat busy room. (notinthedoghouse.com)
The training can then be transferred to areas with increasing distractions such as other busier rooms of the house, the fenced yard and so forth. (notinthedoghouse.com)
One sees it in the selection of artists, with large, busy rooms for performance art by Marina Abramovic, Vito Acconci, and Bruce Nauman. (haberarts.com)