Depends on your interests of course, but MANY businesses, schools, ambulance companies, some amusement parks, stadiums and other sports venues, airlines, as well as some utilities use DMR or NXDN.
There are 2 large areawide business trunked systems that are fairly busy (or at least were before the 'rona) - Fisher Wireless and Mobile Relay Associates. Many TG's are boring (tow trucks, taxis, bus services, etc...) you may find other more interesting interesting TG's not in the DB.