Gog by far.
Haven't tried Amazon's service.
Steam and Origin are drm clients. Origin is bluntly obvious about that.
Steam and Valve as a whole are the group responsible for kicking off this whole online client based drm mess so I have the least fondness or patience for. All the laziness inducing auto-patching and modding portal for the even more lazy isn't going to change my opinion. When Valve start to release drm free then I'll be interested. As it stands they're simply saying - customers can't be trusted and need to be treated as though they are by default pirates. The opposite of gog.