Cornish Cross are the gold standard for meat birds.
Pros: Grows fast, great ratio of turning feed into weight (
2:1 FCR,
#2,
#3, though my personal experience is 2.5:1), great muscle mass. Grow for 6 weeks to get fat birds fast eating feed freely, or grow to 10 weeks on restricted feed for healthier birds with slightly firmer meat. Great personalities, great flock members. White feathers make plucking clean easy. Mild taste.
Cons: Raised on free feed in confinement they can eat themselves to death. While you can raise them out to adults they will only live a few years. Does not breed true. Lays very few eggs.
"Ranger" chickens are an alternative
Pros: Robust birds that are healthier than CX. Will live a longer lifespan, lay eggs more regularly, a little more wily around predators. Will grow out in 12 weeks. Firm meat, stronger flavors.
Cons; Feed conversion ratio is moderate at best and no matter who you ask is easily TWICE that of a CX (
3.5-4:1 FCR,
#2,
#3). Dark feathers leads to more difficulty getting a clean looking pluck. Also a hybrid and won't breed true.
Other breeds
There was
a study funded by SARE that studied not only FCRs for heritage breeds (which on average were 5:1, which according to many reports is a comparable FCR to a "ranger" breed) but also flavor. They raised several breeds side-by-side under nearly identical conditions and calculated outcomes for feed consumption, live weight produced, edible weight produced, and most critically flavor. Their results were super interesting indicating that Speckled Sussex and Domniniques have a better flavor than other breeds.
Similarly, my own experiences have found that heavy bodied, nearly dual purpose chickens, can produce a decent roasting bird in a reasonable time frame. A 12 week old group of buff orpington cockerels I raised out for example included one bird that was 5lbs after processing and they were well filled out and good tasting.
My conclusion, based on my own experiences and the documented experiences of others, is that if you're going to try for something even vaguely economical, just go for the CX and manage them carefully. They do best in big open spaces, restricted feed and/or wet feed environments, where they are required to move around every day and put their voracious appetite to good use foraging. Under these conditions I've personally found them to be healthy birds. Otherwise, I eat my roosters from heavier breeds.