SF has the notorious 38 Geary that had 55+K daily ridership (boardings) in 2017 [1]. It has many of the problems of an overloaded bus line (traffic causing buses to run together, etc). Oddly, the SF Muni metro lines (what they call the light rail, runs underground downtown but on the street elsewhere) don't seem as congested, but I can't find the ridership numbers.

People have been complaining about the 38 for years, but I don't want to criticize the SFMTA (local agency that runs buses/trolley buses and light rail) because it does seem to get the job done without much expensive investment (cost of most recent underground metro in SF was $1.6B for 1.7 miles [2]).

[1] https://sf.curbed.com/2017/9/19/16302396/sf-worst-bus-transi...

[2] https://www.sfmta.com/projects/central-subway-project