It depends on whether you want Dutch process cocoa or not.
From The Art Of Chocolate By Elaine Gonzalez:
Unsweetened cocoa powder is pulverised, partially defatted chocolate liquor. Two types are available in supermarkets: natural (no alkalised) and Dutched or Dutch-process (alkalised), name in honour of the Dutch chemist who invented it, Coneraad van Houten.
Natural cocoa is light in colour and somewhat acidic, with a strong chocolate flavour. In baking, use natural cocoa in recipes that call for baking soda (because it is an alkali). Combining natural cocoa (an acid) with an alkali such as baking soda created a leavening action that allows the batter to rise during baking.
Dutch-processed cocoa has been processed with alkali to neutralise it’s natural acidity, so it is darker (sometimes with a reddish cast), milder in taste, and less acidic than the nonalkalised kind, even though its colour suggests that it is the stonger flavoured of the two. Use Dutch-processed cocoa in recipes that call for baking powder as the primary leavener. If alkalised cocoa is combined with baking soda (another alkali) in a cake recipe, it will create an overabundance of alkali in the batter. As a result the batter will not rise properly, and the cake will have a soapy taste. In recipes where no leavening is required, you can use either one.
Until recently, almost all alkalised cocoa was imported from Europe, but American brands are now available as well. Do not confuse either of these cocoa products with cocoa powder mixes, which contain dry milk powder, sugar and flavourings.
Natural (nonalkalised) cocoa powders include Ghirardelli Premium unsweetened Cocoa, Hershey’s Cocoa and Nestlé Toll House Cocoa.
Dröste Cocoa, Van Houten Cacao and Hershey’s European Style Dutched (alkalised) Cocoa are available in most supermarkets. Bensdorf, DeZaanm Lindt, Pernigotti, Valhrona, Van Leer and Merkens Dutch-process cocoas are sold in specialty food stores and through mail-order catalogs.
P.S. I'm sure Cook's Illustrated has done tests on cocoa at some point. If you become a member for a month, you'll have access to the results of all their tests.