Which OBD-II adapter should I buy?

The OBD-II standard became mandatory in 1996 for cars sold in US market, 2001 for cars sold in EU market, 2008 for cars sold in the Chinese market and 2021 for motorbikes sold in EU (EURO 5). If you have earlier vehicle, or a vehicle sold in other market, please research if your vehicle supports OBD-II before buying a reader. On cars the OBD-II connector is a standard, but on motorbikes you’ll always need an adapter cable for the proprietary diagnostic port.

On iOS:
OBDLink MX+ Bluetooth: Super fast update rate, on a modern car up-to 50 Hz. Has a power saving feature, so it won’t drain your battery if you leave the adapter connected, and a physical button for pairing as a security measure. NOTICE: You’ll need to buy the one with ‘+’ at the end of the model name for iOS, the others will not work.
OBDLink CX Bluetooth: Not as fast, and does not support as wide range of vehicles as the OBDLink MX+, but more affordable and connects using Bluetooth LE.
Carista Bluetooth OBD2: Affordable alternative, update rate at 10-15 Hz, connects using Bluetooth LE.

On Android:
OBDLink LX Bluetooth, MX Bluetooth or MX+ Bluetooth: Super fast update rate, on a modern car up-to 50 Hz. Has a power saving feature, so it won’t drain your battery if you leave the adapter connected, and a physical button for pairing as a security measure.
OBDLink CX Bluetooth: Not as fast, and does not support as wide range of vehicles as the other OBDLink models, but more affordable and connects using Bluetooth LE.
Carista Bluetooth OBD2: Affordable alternative, update rate at 10-15 Hz, connects using Bluetooth LE.

There’s many more adapters that will work, but the ones mentioned are the ones I’d buy. Notice the data rates mentioned are shared between the logged channels. If you need to log 3 channels, and your adapter achieves 15 Hz update rate, it means it’s 5 Hz update rate for each of the channels.