If you're talking about number of flights per day, I don't see why a single, daily round trip to NRT would do much for SEA. My guess would be BOS, which has somewhat-frequent flights to all UA hubs. (Or maybe HNL, but there's no service to IAD.)
If you're talking about number of passengers per day, then maybe SEA wins because of the 777 service.
Again, OP should clarify what they mean both by busiest (# of flights or # of pax), and whether they are talking exclusive to UA-metal, or regardless of UA or not.
And are you talking about locations served only by UA mainline, or UX/Ted? Or Codeshares too?
If it's codeshares, ATL wins
