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.

I use the IQAir AirVisual Pro

http://www.aqmd.gov/docs/default-source/aq-spec/laboratory-e...

I purchased an air gradient. All it required was to plug the ICs into the right spot and turn it in. Came preloaded with software and everything. You can customize if you want but it works right out of the box if you buy pre soldered.

Indoor or outdoor? What do you want to monitor / what is your goal with purchasing sensors?

I bought the Awair dingus just for the Hone Assistant integration aspect. I like it a lot.