Generally it's definitely been double deckers I've seen the worst problems occur with although I do remember witnessing a single decker 42A getting a brick through one of its windows in Partick before. Been on a lot of double deckers over the years with scratches all over the windows and back of seats and graffiti on a few of them. Seems to be better in recent times pre-pandemic compared to when I was growing up though. Used to be alcohol all over the floors of some buses and the empty bottles and cans still there too