In the course of making the cast iron skillet chocolate cakes, I took the liberty of experimenting with quite a few brands of cocoa, did the same with chocolate pudding, and all cocoa will make a cake, but some stand out as remarkably chocolatey. Those, you usually won't find on store shelves they are produced for the high end commercial market and sold in bulk bags of 50 and 100 lbs.
I won't flood the board with pictures, but will list off a few in order of importance, finishing with a couple that stood out to me as excellent.
First off, you have your default Hershey's cocoa, both the regular and the special dark.
The special dark changed recently, they dutched it to make it mellower which to me took out that slight bitterness that used to linger on the back of your tongue. What? But we LOVE that little sour bitter taste that hangs just off the back of your tongue, that's when you knew you were having really good chocolate!
Then there is Ghiradelli, they have cocoa, and dutch cocoa on store shelves, these are great, definitely a step up from Hershey's.
Now you get into your specialty sourced cocoa, ebay is your friend, you can get these broken down and sold in 1lb lots.
Ghiradelli Sunrise Cocoa - very good quality, good results.
Ghiradelli Majestic Cocoa - Extremely good, 22% - 24% cocoa fat, very noticeable finish that lingers.
Gerken's Garnet Red Cocoa - Another extremely good one, 22% - 24% cocoa fat, also a very noticeable finish that lingers.
These two were my favorites, and they will remain a shelf item here.
There are others that I have not tried but would like to, Valhrona, Callebaut, Guittard, Aristocrat, all make high fat cocoa powders, both dutched and non dutched, there are a few more that deserve mention but the ones I listed as favorites will certainly take home the blue ribbon!