I had to smile in recognition when I read this - because Rheingold is by far the one I most frequently listen to, too, and for pretty much the same reason as you. The truth is that for long stretches of time, I'm just not up to the task of absorbing The Ring properly, but Rheingold somehow stands apart from the rest - you can listen to it as a separate, more or less self-contained unit, in a reasonable length of time, can't you? So for me, it's more a case of recognising my own frailty and lack of stamina but wanting a bit of Ring-flavoured excitement, rather than really thinking of it as a 'favourite' section.