I vote for Plant Manipulation.
Electricity Manipulation is very overdone and I don’t really like water manipulation for a character since it’s so hard to write about.
Plant Manipulation would be amazing. The stuff you can pull off with it is astounding. Study some botany and you’ll understand. You can control entire forests, bend the majority of nature (land-based) to your will, you can control the properties of plants which could help you accomplish lots of tasks such as emitting poison from them, or even reversing photosynthesis so that you can absorb the oxygen from the air, creating a vacuum. On a large scale, you can create massive trees and exotic plants and amplify the properties of others like the Venus fly trap.
You could potentially control the moisture in plants which can grant you limited Water Manipulation anyway. You can even create new types of plants (fungi, flowers, etc.) to facilitate everyday life or for combat.
I could go on forever but I’ll stop there. Look up some botany facts, look at the power’s variations and applications, and dig deep. You’ll find pretty crazy stuff.