I don't think Idiocracy is likely. Why? Simple - the world is not confined to just US. There are other countries out there quite willing to do selective breeding to bypass this problem. China for example - they already have child laws. Granted, their one child rule kind-of backfired, but they would totally go for it, if they saw that they could gain an advantage this way. US (or whatever) would retaliate in kind.
The closest we can come to such a future is in a small area that is not interesting to anybody. Like some dumb little island somewhere. Otherwise the society would be taken over, conquered, and you would get the Brave New World scenario, but with Chinese overlords.
The same sort of thing goes for many of the other extremes. While it is possible to arrive at these distopias in a small closed system, in a larger context it won't work. Competition with other governments would either topple this system or produce a balancing effect/effort. E.g. the Brave New World thing - with this many dumb happy idiots, a country with smart people could produce weapons systems that negate any advantages. Or just outpace those guys to achieve complete technological superiority, making the other place look like a 3rd world country.
IMHO the machine distopia is the likeliest. Singularity seems to be approaching - computers keep getting faster and smarter. And our economy is dependent on making them smarter and faster - to do otherwise would be to fail as a company and loose to somebody who is advancing technologically. As a result soon (within 100 years, my guess) machines will be able to make themselvs smarter, growing in capabilities at an exponential rate.
At best we can all become cyborgs and/or transform ourselves into AIs in order to compete. Or perhaps the machines will simply ignore us, outgrowing this planet. At worst we will become extinct or pets. Pets with orgies and drugs, though.