Hardiness or something that is bred in over time. You might have one line of Rhode Island Reds that have hardiness and another line that doesn't have it. They way to breed for hardiness is to cull (remove from your breeding flock) anything that has ever been sick even if they get better and look great after their illness. You don't vaccinate or medicate. You cull for any type of physical defects. You only breed birds that are 2+ years old. You only breed the top 10% of the flock in terms of vigor. Vigor is determined by the growth rates of the bird, mature weights, strength of wings (i.e. do the males hold their wings tucked up tight or hanging low, when you pull the wing out do they snap it back quickly when you let it go or does in retract slowly), activities levels (i.e. do they go into roost and hour before sun set or are they out after sun set up until it is dark, do they forage in the middle of the day when it is hot or loaf by the feed bucket, do the males strut when they walk or walk flat footed, etc.). Also remove anything that has any type of defect, anything that doesn't grow well. Etc. Breeders that breed for hard birds have very few losses after their flock is established (this may take 3-4 generations to do). Oh...and there are different parasites and micro organisms in West Oklahoma than there are in California, Florida, and New York. So...birds need to acclimatize to your property. It is not uncommon for me to loose birds the first year I am working with a few flock. After I get through the first generation and weed out all the weak birds and breed from the strongest birds in the flock it is very rare for me to lose any birds.
So...when it comes to hardiness breed is a lot less important that how long a line has been bred and how it has been breed. Buy from a local breeder if hardiness is high on your list of requirements. Make sure they have been working with their line for a minimum of 3 years (the longer the better) and ask question to see if they are culling weak birds and doing the work required to identify the strongest birds.
Laying ability is similar to hardiness. It comes with the breeding. Their are about 5-6 "primary producers" that supply 90% of the commercial layers. They track every egg that every hen they own lay from its first pullet egg to its last egg. They may go through 100 hens and only breed the top hens on that group. If you want high production then go with a commercial hen. They can be ordered from a hatchery. The White Leghorns are typically going to lay 300+ eggs in their first year of laying. The commercial Rhode Islands Red that I had did not do as well as the White Leghorns but was still laying in the 275 egg range. As far as backyard flocks go the Plymouth Rock is the post popular breed. The hybirds and sexlinks are production birds. The crosses typically lay more eggs in the first year than the pure breeds. This is due to what we call Hybird Vigor.
The average age of a chicken in the USA is 9 months old (if I remember that stat correctly). This is because the commercial layers/broilers are very short lived. Our commercial White Leghorns laid a storm of eggs in their first year but they are breed for one year of production. That means that the primary producers are not waiting 2 years to select breeding stock. They are selecting breeders based on their first year production and breeding them. They do not develop longevity. Any weakness or defect that shows up in their 2nd year they pass o to offspring (they are not fully developed until about two years old despite starting to lay at 5 months old). Many of our commercial hens started to develop problems in their 2-3 year and we never had any survive past their 3rd year. They would get leg problems, internal problems, etc. So out experience was that we could get hard birds if we managed our flock like a heritage farm. The Heritage type flocks topped out at about 200 eggs in their first year of laying but would lay twice as long as commercial hens and live three times as long. Commercial hens would lay twice as many eggs as the heritage type birds but the high production wares the hens out and her laying life is much shorter and in the 2-3 year many of them developed physical defects.
