Okra…grows like a wildfire in the FL heat.
Basil would be the easiest herb both in zone 7 thru zone 9a. Just have to keep up with picking in both. The latter will bolt in you. Okra actually produces the more you pick them.
Oregano. Planted one plant and ignored it. It's spreading and going great guns. And borage is something I planted once and will never need to plant again. It's reseeding itself nilly and willy.
Last year I planted some zinnia seeds and forgot about them. This spring they popped up in one of my mom's planters, and they've been doing very well. I guess that counts?