A few I know off the top of my head:
- DC's busiest line is generally the Georgia Avenue line, which carries about 20,000 riders per day. Sometimes it jockeys with the 16th Street line and the Pennsylvania-Wisconsin Avenues line, which also carry about that many. But the key piece of info is that buses here are topping out at about 20k/day.
- Denver's 16th Street Mall shuttle carries about 45k/day.
- Vancouver's Broadway buses carry about 56k/day
- Cleveland's HealthLine BRT at 14,000/day
I imagine a lot of cities have buses in the 20k/day range, but more than that is probably pretty rare. I know about Denver & Vancouver because they're exceptions.