There are easy-fleshing Angus, but NO WAY would I ever say that they take the cake on it, as a breed average. In general I would say Galloways, Herefords, maybe Luings and Shorthorns. There are great-fleshing Shorties, but on average, I don't think they come close to Galloways and Herefords. JMO. Feed conversion numbers done at Olds College in their Steer-A-Year Trials have proven the efficiency ( = fleshing ability ) of Galloways. They've ranked number one more years than any other breed in the last 20 years.
Besides that impartial testing, I've seen it for myself with our own herd. I've used Galloway bulls for two years, and after seeing what they can do in a variety of conditions, I'm a believer in their ability to do many things, let alone flesh up. They'll do it, and they'll do it on less.