That is a tough one. I would say for the happiest ending I would go with the ending for Final Fantasy VII. Cloud once again became a warrior and was able to get over the death of his friends. As a series I would have to say Rumbling Hearts because in the end no one lost out, and they were able to move on with their lives. Worst endings I would say would be the original neon genesis evangelion series. It did not explain anything, and you did not even know if it was real.