To pick the best, you really need to handle them, measure and weigh them. Checking the sop is a good idea. But other things to check for: Look carefully at their legs, they need to be good and have good feet, their nails should be worn evenly. Touch their breast area, thin ones should be culled. Look at their head shapes, and beaks, need to meet straight together. If you keep good records, generally speaking, heavier and maturity earlier are good signs.
Feathers hide a lot of sins, you really need to handle each one, getting the feel of the structure underneath those feathers. Handle maybe all of them, to get a sense of what you are looking for, separate them how you think, then handle them all again. I would divide into half, then divide the half into half again, and that top 25% would be what you keep, give or take.
It is often helpful to cull in different sessions. First cull, earliest cull, small, thin poor doers. A couple of weeks later, cull again. Rooster genetics will have the biggest impact long term on your flock. Leave two or three if you can as long as possible, but again early maturity, good fleshing and disposition of a rooster is very important.
Mrs K