Dendrobium and Cherry are currently most useful/powerful.
Nerine has the ignore movement effects passive, which is useful. But not really amazing.
Sakura becomes hilariously broken when bloomed, but we shouldn't get that for at least 6-10 more months.
Honorable mention to bloomed Daisy also.
Overall, I recommend picking based on looks. You'll be a lot happier that way. All of the rainbows are pretty strong in their own way.