Angus. I had Angus breed champion, baby beef, six years running at the Iowa County Fair.
Red Angus kicks Black Angus butts.
When it comes to marbling .... Shorthorn comes in a close second to Black Whiteface. Although, the best beef steak that I ever had came from a Hereford.
It's hard to go wrong with any English/Scottish breed.
After backgrounding ... feed 'em HARD on nothing but corn and protein stock for a minimum of six months ... and that's a minimum. Only enough roughage to keep the diarrhea down.