Starrett, the fit and finish is immaculate. The edges are both sharp yet eased enough they don't cut your hand. The numbers are etched, easy to read. The rule is either shiny or satin (your preference), made of tool steel and hardened, flat, straight, correct. The heads are cast or forged (your choice). Cast are heavier, forged are smoother, I prefer the latter but it doesn't matter. Mine are square every time, dead nutz in summer and winter. They 'feel' good in the hand, good balance, and I feel happy using them.
Brown & Sharpe combo square: same as above but the edges are not eased and may cut your hand if not careful. I have a BS double square that doesn't lock up 100%. If Starrett is a "10", B&S is a "9.7".
Lufkin combo square: I have one vintage Lufkin square that is high quality, dead nutz, but is a bit worn. I still use it constantly.
Stanley, Craftsman combo squares: Passable, not always square, have to check it and depending on the season, temperature, whatever; it may or may not be dead square. Not made of tool steel, not hardened. Numbers are stamped and more difficult to read. Even if you try filling in the numbers with ink or paint, the stamps are uneven so the numbers are not always clear. If you move the head, it may require fiddling to make it square.
Stanley Try Squares: Some seasons they are square, some not.
Empire: I have no experience with these. In the store they feel light duty like the Stanley or Craftsman.