If I HAD to keep only ONE breed of laying hen I'd get the Red Sex-Link. They are selectively bred to be egg machines and also to be docile. I've had great success with the ones I've gotten from McMurray, they call them Red Stars. Mine give the biggest brown eggs of the flock. I also have a Black Star from McMurray, I can't tell her eggs apart from the rest of the flock, but she's a nice hen, 4 years old & still laying, still has beautifully soft glossy feathers.
The Black Star roos do look like Barred Rocks, maybe with some brown in their feathers. They make nice meaty birds, taking about 18-20 weeks to reach full size but they don't eat too much in that time. I haven't gotten any Red Star roos, but once I hatched an egg from my Red Star hen, must have been sired by the Brown Leghorn roo, that made a huge red-and-white roo.
Maybe one day I'll keep a flock of just one breed of chicken, until then I'll continue to acquire "one of this, one of that" and have a beautiful mixed bouquet of chickens to adorn my yard.