I know this is an old post but I want to comment on it in case someone else besides me is reading old posts about nystatin. I think that Jorge was misled because of the oral suspension part of the post. Notice that it is a powder so the oral suspension part just means that it is a powder meant to be taken orally as opposed to the powder that is only supposed to be used for topical purposes. It is actually the refrigerated powder that Jorge says is best.
As for the amount of bitterness, my understanding is that as the powder ages and is exposed to more heat, such as room temperature as opposed to refrigeration, it becomes more bitter, so the mild taste should mean that the powder is fresh.