There is no one tablet that is the best. I recommend that you look for devices with a camera if possible. It used to be that the least expensive devices did not have cameras but that is rarely the case anymore. Several App Inventor teachers have found that
amazon.com, Best Buy, etc. have low-cost devices that work just fine.
That said, I invite others on this forum to reply here with the particular device(s) that they have found to work well.