A mountain bike. Every measure of my health has improved and I've never traveling all over to hit bike parks and trails. I've spent more time outdoors this summer than I have the last 5 years.
What brand? I recently got a road bike but honestly don't love it. I think I would prefer a mountain bike.
Expect to spend about three grand for a good full squish mountain bike. I have a Rocky Mountain and I would highly recommend the brand. They are one of the best values in terms of features, weight, and quality. There are even some really good direct-to-consumer brands now like YT. For mountain biking you definitely want disc brakes, a dropper post, and around 120mm-130mm of front travel with a total weight less then 28lbs. Global Mountain biking network on YouTube is a great resource as well as local riding clubs and bike shops. Good luck!