I think you leghorns are really pretty birds and I don't understand the part about wondering whether they are worthy of breeding. What about the fact that leghorns are great layers and you get eggs from them daily.
Who were you talking to?
Anyway.... if a bird is worthy of breeding... you need to look at all of the birds that you have and first see if they have any big problems (actual conformation problems like a bent back, bent toes cross beak), since we are talking about Leghorns, you probably want to make sure that the bird you are looking at lays very well and lays large eggs.
etc. etc etc.
After all of that stuff, you will also want to look at the feather color, leg color, the comb size and shape, etc.
There are many things to look at before you decide which birds you want to breed, or if they are worthy of breeding.