Best Instant Hot Chocolate or Cocoa
There is no way to make good cocoa by putting powder into hot water. Good cocoa is made by heating milk (low heat is essential; use a double boiler) and adding to it either cocoa powder and sugar (Droste is probably the best) or sweetened dark chocolate broken up into small pieces.
Powders like Swiss Miss and Carnation and (I think) Nestle are cocoa powder with milk powder and sugar. They are much too sweet and have very little chocolate flavor.
Jeremy's response notwithstanding, I had decent luck with some packets (I think they were legal)I brought back from Spain two or three years ago. The stuff didn't compare with the incredible cups of dense hot chocolate served in little cups in Spain, but was certainly better than anything I've had here. If there are any stores near you selling imported Spanish foods, you might want to check them out.
An interesting alternative, which makes an incredible sauce for ice cream, is to put about half a cup of chocolate bits into a microwavable bowl (I prefer the Ghirardelli double-size bits, but Nestle's will do), a heaping tablespoon or two of Ghirardelli (or other brand) premium unsweetened cocoa, and water to cover.
Nuke the stuff for 3 minutes at .30 power uncovered, then whisk it until smooth. If too thick, whisk in a little more water until it reaches the consistency you want. I sometimes add a tablespoon of scotch or cognac to vary the flavor. The whole thing probably takes no more time than doing the instant thing.