You should select a board that suits the physical constraints of your vehicle, the activities you wish to perform, and of course cost.
Pixhawk Series open-hardware flight controllers run PX4 on NuttX OS. With many form factors, there are versions targeted towards many use cases and market segments.
If you need computer vision or other computationally intensive tasks then instead consider a board with companion computing.
|mRo Pixhawk||Popular general purpose flight controller (this is a slightly updated version of the discontinued 3DR Pixhawk 1).
Also consider: HKPilot32, Dropix, mRobotics-X2.1.
|Pixracer||Very small/light autopilot optimised for FPV racers. It is suited to any small frame that requires no more than 6 PWM outputs.
Also consider: Pixhawk 3 Pro, MindRacer, Pixfalcon.
|Pixhawk Mini||Small general purpose autopilot that has been optimised for ease of setup.
The controller has internal vibration damping and only 8 main outputs (no AUX ports), making it much less daunting to install and connect. It is not suitable for vehicles/functions that require AUX ports.
|Pixhawk 2||Flexible autopilot intended primarily for manufacturers of commercial systems. It is designed to be used with a domain-specific carrier board in order to reduce the wiring, improve reliability, and ease of assembly.|
|Pixhawk 4||Pixhawk 4 is optimized to run PX4 version 1.7 and is suitable for academic and commercial developers. It features more computing power and 2X the RAM than previous versions, additional ports for better integration and expansion, new sensors and integrated vibration isolation.|
These flight controllers (and development platforms) offer on-vehicle "companion computing", enabling computer vision and other computationally intensive tasks.
|Qualcomm Snapdragon Flight||A high-end autopilot computer that runs PX4 on the DSP (on QuRT RTOS). It includes a camera and WiFi.|
|Intel® Aero Ready to Fly Drone||A UAS development platform that integrates a Linux computer, PX4 on NuttX, and a camera in a single package.|
|Raspberry Pi 2/3 Navio2||RaPi can be connected to an autopilot and used as a companion computer.|
PX4 is available on many popular commercial drone products, including some that ship with PX4 and others that can be updated with PX4 (allowing you to add mission planning and other PX4 Flight modes to your vehicle).
For more information see Complete Vehicles.