The biggest and best investment right now are fiery brimstones. Market rose and fall, always make a hell load of gold on brimstones every patch with idiots always failing to sell them before patch hits and prices plummet.
Blizzard just made a big change to brimstone usage and kinda pledged to introduce more ways to use brimstones. Prices only went down because of the massive stockpile of brimstones from 1.01 till now. Prices are almost rock bottom these days and grab them if pricing is on the low side of daily fluctuations. It isn't hard to see why brimstones are the best investments now.
On average you find about 5 demonic essences in a normal run...and maybe 1 legendary every few runs. All essences are turned into gear and this will continue for a long time and brimstones are quickly eaten up by the system. If you have been cashing in on brimstones when prices are low lately you will definitely see a huge profit in a few months. If you are going on a long hiatus, I will reccomend you change your gold into brimstones (at a good rate of course) and come back later to resell.
Personally i bought in quite a fair bit when prices were 7k per brimstone and now they are up to 23k....easy moneys.....laters people...