yeah, waitrose does very nice cakes but their price is not that much cheaper than M&S
The "Morrisons The Best Chocolate Cake" gets mostly positive reviews and I found it very moist when I tried it recently (unlike some of the very dry cakes from other supermarkets). The main recurring theme amongst critics is that it's more chocolate-fudge rather than pure chocolate. But perhaps that makes it better for a kid's birthday?
M&S remain the best for birthday cakes in my opinion. Not being dry is & jams & creams are tasty are the stand out reasons. Other ranges like their coffee cakes or chocolate fudge etc can be matched by other supermarkets but birthday cakes remain above the other supermarkets.
M&S are yummy.