Since I believe you are only referring to large fowl I believe the black variety should be the "go to" variety for most outcrosses within the breed. Blacks are the closest to the Standard when it comes to type. Their combs are better than those of the buff and silver varieties. Their eye color (of course some lines/strains are better than others) has greatly improved in the past several years and many meet the Standard. Their large to extra-large blue egg color is unsurpassed and that has only happened in the past decade for the most part. Some have mentioned they think are large fowl Ameraucanas are getting too big. Some are over the 20% leeway allowed by the Standard, but a decade ago I purposely increased their size because some were undersize. Now they average closer Standard weights plus more often than minus. Their tail angel is generally correct. Multiple spurs an mature males is something to watch for and not breed from.
I've made the LF buff/black outcross at least a couple times and seen improvements, but it takes several rather than a few years. LF buffs need improvement in egg size and color and combs. Their soft feathers leave females barebacked very often, but that may just be something that goes with buff feathered LF. Their shank color used to be too light, but I had nice slate shanks on them the last several years.
Bantams are different story.
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