Challenge Mode boosts. 20-30k per hour(depends which CMs and if we wipe to some silly stuff) with just a guild group of people who know what to do. We do Tank, Healer, DPS as the ones who are boosting(and getting the gold!), then 1 guildie/friend DPS who gets xmog and then the boosted DPS.
I guess it would be hard to pin point the exact task that can make the most gold because its going to be situational in every case.
boosting ppl or farming and or playing the AH, are probably all the best ways of making lots of gold, it still all comes down to time=/=effort = reward.
i personally think that levelling multiple toons, taking them all through the timeless isle running them all through the lfr will more than likely net you lots of gold, from what i can tell going from 1>90 earns you round about 5k gold at least (its probably nearer to 10k), do that 10 times and you have 50k gold along with one of every class at max level < and that is the bit that is worth more than any amount of gold.
I've played for years now and i have most classes that are at least 85, nearly have 4 90's, but my gold these days seems to fluctuate between 22k and 19k, over the last month i've gone upto 22k then back down to 19k, 3 times (maxing 3 crafting professions will do that) i do wonder how long it will take to earn the 100k for the yak, i think in that case it really comes down to not spending any while your saving for something. like if you keep enchanting your gear on your alts and buying gems and such, you probably won't make gold very fast.