Honestly, it is pretty hard to give exact numbers (especially with how people travel around) but I would say the bulk of characters can be found in Johto most of the time. Most newcomers also choose to first hit up Johto. So if you want the most bustling region, it is definitely Johto.

(That said, for the big events, we generally try to arrange some form of transport so even the most far flung characters can have a chance to join in)