My current favorite is pickerel weed. Here's why:

1.) great at pulling nutrients out of the water helping control unwanted algae
2.) attracts dragonflies and pollinators with it's flowers
3.) spreads but does so at a slower rate so management is doable
4.) doesn't mind water fluctuations.

I've got a patch at our pond that I can't wait to separate and replant in different locations once the weather cools.

Ephesians 2:10