A once in a match move is never going to be ranked the best, ever. If it has such a specific use, then its not good. I'll use Kirby as an example. Kirby has one of the best bairs in the game. It has good KO potential, high priority, low lag, and can even WoP at low percentages. Kirby has one of the worst dash attacks in the game (it looked good at first but it really sucks) assuming that you hit with it, it can be easily DI'd out of and leaves very noticeable lag. In the case of Dedede's dash attack, it's not good by this logic. Though the knockback is excellent, it suffers from a large amount of start-up and cool down lag. This means that he may not even get it off, or he will get punished afterwards. Falco is also a great case of a good dash attack due to dash canceling. It can lead to combos, be dash canceled and even if shielded, a dash cancel will push you far enough back to ensure Falco's safety from excessive repercussion. The specificity of Dedede's dash attack makes it a very bad choice in most situations and therefore a bad move.