Busiest? Almost certainly.
Most difficult? Nah, that is not really full air traffic control separating traffic, it is VFR "see and avoid" with lots of options open and procedures in place to move the traffic.
I expect the most difficlut would be any busy TMA when weather avoidance starts during an already busy delayed period and no place to go. How many would be on frequency then? I don't know, but it will feel like too many.