You have the potential for a larger profit on the auction house.
And long term, that is probably the best place for the largest profits.

Transmutes can be very decent too, depending on how many are on the market.
And the demand for the items.

I find the dailies to be the easiest.
In that it is a fixed amount, no matter what the market is like.
It is one of the more boring aspects of wow.
And running dailies on three sets of five toons gets old very fast.
But it is consistent, cannot be undercut etc.