I think the best fairy types are Azumarill (you can catch Azurill in route 3) and mega mawile(you can catch mawile in glitering cave). They both are very bulky pokemon who can take heavy damage, but at the same can land a strong hits. MegaMawile is much more powerful but Azumarill got Aqua jet which can help him with his slow speed.
I heard Florges is a pretty strong fairy overall. Also I'm a little biased but Mawile served me well in the story and its mega form will only help it further.
Competitively I'm not sure, probably Gardevoir.