So no one is willing to share their methods?
Framac
I missed your post, am really in catch-up mode!! Not sure who all replied to this question, it's a very good one.
It should change as you go through the years. For example, though we like a long-back in Giants, you certainly shouldn't choose for that attribute year after year, as you'll end up with a very unbalanced bird eventually.
First, a Giant must have a horizontal, long back. A short-backed bird with an uphill (or downhill) back doesn't make it to my breed pen. Also, I look for a good amount of width between the legs. If I can't put my entire fist between the legs at the top when they are a year old, they're culls. The bottoms of their feet must be yellow. Wing carriage should be horizontal. Eye must be very dark, almost black. Once these criteria are met, I go on to things like:
Size & shape of skull. I want a bird with a good width from top of skull to top of eye, and bottom of eye to bottom of skull.
Overall balance. A bird that has good leg placement - not forward, or back, but right in the middle. Blues should have nice edging on their feathers (folks, this is a term commonly misused; edging is the same color as the base, only darker; most folks call it "penciling" and that's not accurate. Penciling is black). I also have been looking at feather quality for a few years now and have learned a ton just this year from a judge in reagard to feather quality. My Giant mentor has pushed this with me, and I'm SO very glad of it!
Length of tail. In my flock, tails tend to be just a hair shorter than I like, so I'm working for a bit longer tail this year.