I'm usually a contrarian on these threads but Edinburgh is pretty hard to beat. And it knows it.
London of course - for every grotty bit there's usually at least one or two absolutely stunning places and it has unexpected beauty in odd places - the gorgeous playgrounds around the Holloway Road, the view from Suicide Bridge, the top of Centrepoint when the sky is blue...
I will defend to my last days Liverpool being beautiful. The Liver building, the Anglican Cathedral and all the neo-classical galleries. Stunning.
I don't get the love for Oxford or Cambridge - some lovely historical buildings in otherwise blah cities. It's like saying Peterborough is beautiful.