I'm taking overused as meaning more used despite it not being as good as others.
I will have to go with Aerospray and maybe the krak-on roller.
Aerosprays are just good at spreading a lot of ink in a short time, but their lack of range is a problem. And yet they are used quite a bit. I think it is overused because of its user-friendlyness with how good it is at inking turf.
Krak-on Roller...Could be good, but I think the main thing about this Roller is that it is mainly used also because of user friendlyness. People like how the Kraken is a good panic button, and it being a regular roller helps people like it more. It isn't lacking in damage like the paintbrush, slow like the dynamo, or lacking in good subs and specials like the regular.