According to a press release from NASA …
Data from satellite sensors show that during the Northern Hemisphere’s growing season, the Midwest region of the United States boasts more photosynthetic activity than any other spot on Earth, according to NASA and university scientists.
They determined this by measuring the fluorescent glow that healthy plants give off when they grow. It is not visible to the human eye but can be picked up by special sensors on satellites. The press release has a lot more details.
If you click on the map, you can see the full version. While they don’t have any state boundaries, you can make out Lake Michigan. Based on that, it looks like one of the brightest areas is across central and northern Illinois – no surprise there.