For me, the best event girl over all the others is clearly Mountain Lily. Her +20% ATK turn one is unique and better than any gacha gold similar ability and once she is bloomed she is a strong counter girl, better than most of gacha counter girls without equipslot unlocked).
Behind her, Echeveria is really good too for similar reasons but her permanant reissue will happen in a looong time so, she can't be an option.

And then, girls like Hypericum, Mirabilis or Geranium are good choice to start and can be helpful girls for a long time.