The benefits are related to how easy it is to reach. That, in turn, relates to what sorts of miles count. Some of these are listed in the post just above this one. In other words, "one million miles" does not have a universal definition, so comparing what different airlines offer for reaching their version of it is a bit pointless. It's almost like asking "in which country can I become a millionaire most quickly?" without taking currency exchange rates into account. You can earn a million Chilean pesos much more quickly than a million £ or euros, but they won't buy you nearly as much after you do.

So, it depends on what you want. If you want a few lifetime benefits with minimum effort, go for AA. If you want more and are willing to fly more, go with someone else - but most other airlines' 1MM level is more comparable to AA's 2MM in terms of what it takes to get there. (Not exactly comparable, since everyone's ratio of BIS to non-BIS miles is different, but more comparable.)