Glasgow Celtic have some scummy fans - I work with some.
Glasgow Rangers have some scummy fans - I work with some.
For every scummy Celtic fan there is a scummy Rangers fan.
For every scummy Rangers fan there is a scummy Celtic fan.
Those two clubs are not typical though. They both represent, still, religious groupings and national identities. That takes them above and beyond clubs merely representing a city, a borough or a town.
Feel free to claim Glasgow Celtic are the biggest club in Britain, the irony being that their fans by an overwhelming majority don't want to be part of Britain.
As it happens any decent Glaswegian would be a Partick Thistle fan as it is they who have graced Moss Lane.