AA, because "Gold" is entry-level there.

At some other airlines, "Gold" is the second level up, which means you need a lot more miles.

Not only do status level names differ between airlines, but what you get with each level of status differs a lot. You need to explain much more, including the benefits you're after. The answer might be quite different depending on whether you want status mostly for upgrades, or mostly for lounge access, or mostly for access to preferred seating (eg, exit rows), or mostly for bonus mile earning on flights.

Also, are you interested in a worldwide answer, or is there a particular city you always start your travels from (which would restrict the airlines available to you)? You need to provide such info to get a practical answer.