I don't think I could disagree with anything said before, but I have a few things to add
For females,
They need to have good wide hips and pelvis, That I find of utmost importance, because if they can't deliver a calf, no other trait is going to matter anymore.
A good udder, because the calf shouldn't have a hard time finding the teats, and will need to have good milk to make it grow
Good legs and hooves, they need to be able to follow the calf and graze, and sometimes they'll have to stand around for 3 or 4 months.. Hoof trimming is a PITA.
Then you have all the other things that make them look nice... I certainly appreciate a nice topline, but I usually look at the bottom line first... a deep chest that stays about level with the ground right back to the udder is what I like... On a young cow, I like the bottom of the teats to be about level with the belly, and that usually makes it so as they get older and things start to sag, the bottom of the udder would be level with the belly, which is still OK for a calf to find. As long as we're talking about beef animals, they have to show muscle, the best cuts are on the back and hind end, so you want to be sure that's where they carry meat.. When looking at calves in particular, I like seeing a "rectangle" when looking from the shoulders to the hips where they are of uniform width... this is reduced with age, but it sure makes for nice looking calves.
For bulls, pretty much all the criteria for heifers applies but I'll add a few
Firstly, I don't like sharp corners anywhere on them... a long smooth head.. Frog eyebrows are a dealbreaker for me! Smooth, broad shoulders and that same "rectangle" I mentioned for calves down the back. I like when their spine is below the back muscles.. He should again have balance between the front end and hind end, I find many bulls look front heavy. He should look masculin, and have a good set of nuts and a tight sheath too.
I've probably forgotten a bunch of things I look for... and others will put importance in different areas...