This may be obvious, but usually all events that are featured by Infinite Flight typically get the most attendance with no gate assignments so there really isn’t a way to tell from those. Feel free to make some of your own events and I’m sure you could get a great deal of pilots if that’s what you’re interested in.
I’m not 100% sure of the largest events that have been held on the IFC, however some of the largest events I’ve seen over time has been the events hosted by the Regulars of the community, with some of the events reaching over over 100 members signing up, and several hundred more on the day of the event (thanks to ATC and heavy traffic causing large influences)
As a regular, I often hosted IFAE’s public events. One of those was the “Flight For Change”. It was thought up by both me and Art Martinez, the CEO. Pilots can choose to donate to cancer research while being a part of this event, and we attracted over 150 pilots. It practically filled up all of Chicago. That day was amazing.