Extreme oversimplification =(
The best fairy depends both on the situation at hand and your own play style.
If you're over geared for the content aka pretty much anything but Savage Alex now, it really doesn't matter, use the one you are most comfortable with and will remember to use!
For non trivial content then it starts to matter. A good way to think of it is to go with the fairy that compliments your playstyle as well as the content you're up against:
If you're learning new content and haven't really got the confidence in yourself (Or your fellow healer) to go into cleric stance then chances are Eos' heals will just be unnecessary and wasteful whereas Selene will bring a DPS bump to the party as a whole.
On the flipside, it's worth weighing up the MP savings via Eos particularly if you are DPSing significant portions of the encounter. If you can reduce/remove the need for ballad via handling those MP hungry AOEs with Eos alone you can not only increase your personal DPS, but the BRD/MCHs as well, this sort of optimisation will usually bring far more to the table than mindlessly going with Selene because.
TLDR: There is no best fairy, choose the one that suits both the situation and you.