Krispy cremes are damn good, I love working in abingdon, the tescos there sells them, I can sit and eat 12 of them in one sitting, but my heart hurts afterwards.
I've driven to the actual store in oxford before just to get some because I was really craving them.
Apart from them, morrisons i find do the best jam donuts, really moist and doughy. Tescos and asda always have very dry own brand ones. Sainburys the jam is too sweet and chemical tasting but they do good custard wheras morrisons custard is a bit tasteless. Morrisons are best for donut quality, jam and apple. Sainsburys do good custard and toffee.
Co op donuts have too much sugar.
To sum up my view on supermarket donuts.