No, I would not say free verse is more powerful. I would also not say structured poetry is more powerful. It depends on the skill of the poet, and the word choice, and the topic. Any form can be powerful.
For writing a “hard-hitting poem,” what can I recommend? Start by writing out some of the things you want to say just how they come to you. This could be in a list format, or a thought cloud, or whatever works for you. Put down everything that comes to you. Write more than you’ll need.
Then read it over and choose the “best” bits. Put those into a poem. Whatever form you think they fit into best.
If you are passionate about the topic, that will come through and only you can write this poem the way you will write it.