Best Event Girls in terms of raw stats are Lavender and Edelweiss.
Dipladenia is also not a bad choice because she has an unique Ability.

You won't find an Event Girl that is equally good compared to a Gacha Girl though. Not on Nutaku or DMM.
Generally Event Girls are tuned down Gacha Girls but with the access to easy Skill and Equipment Slots.

Don't know about any other good Event Girls. Maybe the others know a few hidden gems?