I'm partial to the secondary graffiti that accumulates over time when half-hearted attempts to cover up graffiti with leftover paints seem to create a whole new mural of its own.
https://www.designboom.com/art/brian-kosoff-graffiti-abstrac...
https://www.designboom.com/art/brian-kosoff-graffiti-abstrac...
https://en.m.wikipedia.org/wiki/The_Subconscious_Art_of_Graf...