An interesting question, but I think it’ll depend on the mark you’re measuring with. Something like the Far North Main Line will always have a very low number of physical trains per day, but each one would be relatively well used. Other lines could be the opposite, maybe like the Romford - Upminster branch in London, which is half hourly all day long, but each train will have plenty of empty seats. So the percentage of seats filled out of the daily amount offered may be higher on something like the Scottish Branch Lines, but Romford - Upminster blows it away in terms of service frequency.
Ultimately the passenger line winner would probably be something like the Stockport - Stalybridge line, with its weekly one way service.