Not sure if it's the best but I recently bought a "Airthings 2960 View Plus - Radon & Air Quality Monitor". Choosing the correct app and setup was a bit annoying, but not too bad. I think setup was about an hour once done. Also the phone app wanted to read results for a week or so before displaying some info. The device has a screen that will show you the current values.
I purchased the device because I was a bit worried about radon and other garbage that I might be breathing while working in my basement office. Radon and other data points were not a problem. Turns out C02 was more my problem. It's pretty surprising how quickly that number rises with a closed door. I believe I can attribute the tiredness with the C02 levels. Having history correlates with when I'm in the basement office, and opening a window, it's a walkout basement and I have windows, decreases C02 stat pretty quickly.
Overall I'm happy with my purchase, and I'm considering buying another one for the ground level of the house.
