In general terms -- and this is just MY OWN observation; nothing meant pro or con against any specific facility:
If you consider where the biggest, busiest, most popular airports are in the US in real life (JFK, BOS, ATL, LAX, DFW, DEN, DCA, MIA, MCO, ORD, etcetera), those places are also big, busy, and popular on VATSIM. (Of those, presently, I would contend that Chicago is the one which is a bit understaffed relative to the others right now, but again, that's just my perception based on what little attention I pay to who's online often and who isn't.) So, one strategy might be to look at ARTCCs which do NOT host any of the "big ten" I mentioned above.
If you REALLY want to dig into it, each ARTCC publishes a Controller Roster, and you can look at each to see who has the largest number of controllers. That might be a bit deceiving because different ARTCCs might have different guidelines for keeping active controllers on the roster and removing inactive ones. Some ARTCCs also post stats on controller hours for a given month, year, or other period. That, too, can be skewed for example if a facility has a ton of newer controllers racking up DEL, GND, and TWR hours but not as many on APP/DEP or CTR.
In short -- there's no easy, cut-and-dried answer. But, those thoughts might give you an inkling on how to decide. Best wishes and good luck!
