I think my favorite were Eastern and Geek festival.

The girls overall in eastern were my favorite even though i didn't get a single one on all my 3 accounts. Not very lucky for this one. But thanks internet to let me see what i missed.
But festival geekette is my favorite single one of all the event girls so far. Hard to say why but she is so cute. Thanks for the artist on this one.

As for the event, i think there are way too many now. Getting the regular girls is very hard when you are not lucky and during event i cannot lose energy on regular one when there is a chance to get an exclusive.
There is always the kobans and i was on the point to buy some for the festival event. But man, the price for just a drawing is imho way too high. A balance should be done in that regards.
Anyway the good side of the event is that it allows us to get kobans though the quests. And that is a nice move.

As for future event, why not make possible to get, as in Osawari Island, ticket to get old event girl no longer available. Meaning, you complete some quest, and at the end you have one ticket whick let you roll on the list of girls you do not have and who are no longer in the game because tied to an event. Or something similar. Not very broken and it could give an hope to new and old players alike to get a cute new girl to their harem.

That was my 2cp