What's the simplest and cheapest OBD unit?

[Tomarch]

What is the best/cheapest OBD unit for my 2017 non-REX vehicle?

 

[alohart]

I assume that you're asking about an OBD to WiFi or Bluetooth adapter. If so, be aware that not all of them work well with apps like BimmerCode, BimmerLink, mi3, electrified, etc. The Vgate iCar 2 OBD to WiFi adapter that I bought in 2017 continues to work well with these apps and is available from Amazon at one of the lowest prices for such adapters. I prefer WiFi to Bluetooth because of its greater bandwidth and range, but neither is particularly important for use with these apps, so many are happy with Bluetooth.