This comes with two solutions. Both answers exclude specially named dragons, as their CR rating accounts for them being at the level of deities or acting as a monster of legend. We're only accounting for Dragon Types here. That isn't to say it isn't possible. The most common dragon species with the highest CR rating is unanimously the Greatwyrm Gold dragon. This is because they are considered the strongest of the metallic dragons and hold a revered reputation among their kin. However, suppose you want something rarer but stronger. In that case, a species is only outmatched by a select few named dragons.  Prismatic Dragons are a rare Gem Dragon breed capable of splitting the magic weave itself into a frightening prismatic breath, which consisted of various colors that all held a different power. This, of course, would be removed. In addition, other powerful inclusions to a Prismatic, such as its massive 30 strength, would all stray, making it the contender for the strongest shadow dragon species possible.  To put it in perspective, let us compare stat blocks.  In fifth edition only an Ancient Gold Dragon has a stat block; it stands at a CR 25. An official stat block on an Ancient Prismatic Dragon is difficult to find. They exist in the Canon Lore but are unanimously considered too much of a pain to function. However, the ones I have found place an Ancient Prismatic Dragon at around 30 to 36, depending on the source.