If you can get genuine columbian marans (not made by crossing with a sussex, but the old type) i will eat my shoe

they're the rarest! Black coppers usually lay the darkest, if you live in america chances are low you'll actually get good dark colour. Most wheatens are quite good, but some lack legg feathering. Cuckoos will probably be the worst quality, a lot of people have something that they call cuckoo marans, but they aren't. They lack leg feathering and their egg colour is close to what a regular laying hen lays. I'd stick with the black coppers, that's your highest chance of haing good egg colour and a good quality chicken