Not sure if this is what you're looking for, but here goes:
Healthy (no snotty noses, no droopy ears, no constant squirts of diarreaha, no coughs or hunched backs. Also try to avoid ones that are bouncing right off the ring walls...seems like the high headed ones have a harder time gaining as they're too busy looking for that hole in the fence to waste time getting fat.)
Besides that:
Thick, deep, long-bodied animals (breed choice would be dependant on locale, as in some regions cattle with ear are hot, in others they're not, same for buckskins, blacks, etc)
Also nice if you can get a run-down on the previous owner's herd health regimen. Ie. Vaccinations, deworming, etc. You don't want to waste your hard earned coin on a bunch of pot-bellied, louse ridden calves. You'll be getting them on the road to health before you get them gaining properly, and that's not cost efficient.
If you're in the northern climes, watch for really short ears and tails (a calf that froze his ears and tail at birth may also have frozen his feet, and it makes them poor gainers in the feedlot in most cases.)
Hope this is of some help. Good luck with your venture.