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.
|CUAV V5 nano||Based on the Pixhawk FMUv5 design standard and optimized to run PX4 firmware. Small enough to use in 220mm racing drones, but powerful enough for almost any other drone use.|
|CUAV V5+||The board is based on the Pixhawk FMUv5 design standard, the external interface uses the Pixhawk standard pinouts, and the modular design allows the users to customize their own carrier board. The autopilot is compatible with PX4 FMUv5 firmware and can be used for academic research and commercial systems integration.|
|Holybro 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.|
|Holybro Pixhawk 4 mini||Pixhawk 4 mini is designed for engineers and hobbyists who are looking to tap into the power of Pixhawk 4 but are working with smaller drones. Pixhawk 4 Mini takes the FMU processor and memory resources from the Pixhawk 4 while eliminating interfaces that are normally unused. This allows the Pixhawk 4 Mini to be small enough to fit in a 250mm racer drone.|
|Hex Cube Black||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.|
|mRo 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.
|mRo Pixhawk 1||Popular general purpose flight controller (this is an FMUv3 version of the discontinued 3DR Pixhawk 1).|
These flight controllers (and development platforms) offer on-vehicle "companion computing", enabling computer vision and other computationally intensive tasks.
|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.