+ Wish
Now hear me up, Chansey does NOT learn Wish, only a random event gave it to her along with other random mons that make no sense learning the same move.
Stats + Typing: Chansey and Blissey have the hugest amount of hp in the game, you know how great Wish is on those two since we had access to it on simulators.
Design: They're eggs, but healing-nurse-thingy eggs, all their moves turn around supporting the team by healing them, they even learn Healing Wish, how could they not learn regular Wish? ,similar monsters like Audino and Clefairy and even the other egg-shaped mon, Togepi, learn it, so it makes sense.
Need: Almost no one (I'll bet that actually no one) have a legal Wish Chansey/Blissey on their cartridges, at least, not with good IVs, while it would not affect showdown/PO battles, it would be great for those who play wi-fi. As long as it is not an egg move, Chansey would not lose Seismic Toss from the Emerald tutors.
+ Roost
Stats + Typing: Psychic isn't exactly a great defensive typing, but her defenses are really really good, reliable recovery will allow Cresselia to support her team more easily, and with Reflect up, she take pitiful damage from powerful SE attacks like STAB U-Turn from CB Scizor
Design: She flies doesn't she? I mean, I know she levitates, who we can see her flying in the games, and besides, she's a duck! it makes perfect sense IMO.
Need: Reflect + Toxic + Roost = mini-Lugia