you can have great covers that have little or no relation to the music within, but what do you reckon are the LPs where both tracks and cover blend well?

I'd say the sepia snowstorm on Public Strain just a great job of conveying the thick, ugly beauty within. FlyLo's Cosmogramma is an excellent cover and works properly well in context of the music. Equally Merriweather Post Pavillion's quite an obvious one.