The answer to the question as asked is American.

But it really isn't a very meaningful question... "Gold" is the lowest FF status level on AA, the middle one on CX, and the top one on BA.

If what you meant to ask is "which airline requires the least miles to reach the OneWorld top tier" then the question is more complex. It depends on your flying pattern, both in terms of fare classes flown and lengths of flight. Each airline has different parameters for status levels, but there is no simple answer as to which is the easiest. It has to be said though that CX is probably the hardest on almost any travel pattern.

By the way, in OneWorld while each airline has their own names for the different FF levels, there is a common terminology for "OneWorld status" which goes from "ruby", through "sapphire" to "emerald" (the highest).