What makes Clefable good and usable in every Gen?

While Clefable hasn't exactly been an OU Staple in the early Gens, It still does well in RU and UU and its even banned from GSC UU.

Before being a Fairy type, the lowest it has been is in RU while having niches in OU and UU.

Now to this day its still an amazin Pokemon

What makes Clefable a decent Pokemon to use in all Gens?