Suggestion for platform to work on Autonomous drones

I have been working with Naza M lite and Arducopter APM 2.6 board. However I wish to work on autonomous drones hence I need suggestions on the best platforms available for developers and hobbyists.
I found this Qualcomm SnapDragon platform and there were suggestions within that page.
Any recommendations?

To help you decide, I suggest you should also look into the concept of a Companion Computer. In this case you can use an existing Flight Controller for the low level tasks of reading sensors and controlling actuators ( You could use your APM, though a Pixhawk might be easier, since APM is only supported for older versions of the ArduPilot software). The Companion then runs higher level tasks such as collision avoidance or vision or whatever custom research interest you have. You can start quite cheap, for example with a RaspberryPi as your companion, then once you have the basics sorted out, you could move on to a more powerful companion, such as TX1

This looks interesting. However if I go for a Raspberry Pi as a companion computer, is it possible to read the sensors of PixHawk during flight, for example read accelerometer and gyro sensors so as to prevent drift after moving from point A to point B in a small closed environment like a room? Also can I issue commands to the flight controller from the Pi to correct instability or drift?

The usual way to start with a companion computer is to connect a serial cable which communicates via Mavlink.
The sensor outputs (for example) can be read over Mavlink I think.

If you look at the available messages at MAV_SYS_STATUS_SENSOR section, you can see what is available

Mavlink can also be used to send commands to the flight controller and ultimately MavLink can be extended to your own custom messages.

Cool. I shall try this then. Do post any other suggestions if you have some. My final goal is to run motion control algorithms on the drone so as to achieve stable autonomous flight.