I've spent four years in the UK. No thanks. Some of it is beautiful, the majority is worthless.
Street artists have have designated public spaces for tagging, and friendly private establishment owners can open their structures for tagging too. I don't see why random places should be open for vandalism by random schmuck.
To each place its own way I think. I like the orderly poetry quotations on the walls of buildings you have in Leiden, or the savage, well executed street art you see in out of the way places in Valencia, but even if both of them have quality, neither would fit Singapore in my opinion.
Actually, in what regards graffiti... the only example I saw of that brand of vandalism in Singapore when I was there a few months ago was...
...curious to say the least.
I'm not sure whether it looks more like the ramblings of a whole discombobulation of lunatics or the coded messages from some underground resistance.
Explanations not involving lunatics or conspiracies via private message are most welcome.