It's been a while since we started this thread and in the time since some stuff has changed, other things not so much.
Flight control systems have made it a lot easier to be able to consider different options. I chose the NAZA when I built my quad for this thread (I actually used a NAZA 1 when the NAZA 2 had already been out for a while) because a friend had one that he didn't want anymore and it just ended up in my shop along with the rest of the airframe. I still fly it (was flying it Thursday afternoon as my FPV trainer actually) and it's a lot of fun with a ton of flexibility to try different things. With time though we've got more options to consider, some new, some just refinements on what was already out there.
I recently tried the NAZE32 flight control board and it's great. It's small, has a good amount of support, and flies really well. There are a lot of options to expand the deluxe version with GPS and other add-ons including telemetry. I'm flying the most recent revision, Rev5 so be sure if you buy one used in the classifieds that you know what you're getting.
A development from the NAZE32 is a firmware working group called CleanFlight and that team has released their own flight control board called Seriously Pro Racing F3, or SP Racing F3 for short. I've got one but haven't flown it yet, it also has a basic and deluxe version with the deluxe version offering many add-ons (GPS, telemetry, sonar for low level altitude hold accuracy, etc.).
The KK boards have continued to evolve as well with options available for different firmwares. I'm not as familiar with this system but I do know the hardware is cheap and they fly fine so they ought to be a consideration for anyone looking to throw together a quick and dirty copter to learn the basics on. If anyone is very well versed on the KK boards, please feel free to chime in and share your experiences with us.
3D Robotics continues to develop the Arducopter platform with the APM 2.6 and Pixhawk systems which are similar, but different. These systems probably represent the ultimate in expandability with enough options and add-ons to keep you busy for a good long while. Or you can use them without all of the doo-dads and they're good fliers that are adaptable to small or large platforms.
NAZA-M V2 from DJI is still a great flight control system with a fair amount of fleibility, FWIW.
And don't forget to look in the classifieds as the older systems can usually do as much as the newer systems.
- Hoverfly PRO is the best flying flight control board I've seen. They are bigger than the rest so they need some room on the frame for mounting but they are perfect in how they fly and they are dirt cheap in the used market. They also do awesome with cheap chinese ESC's so you don't have to splurge for the latest tricks to get excellent control. The GPS never quite lived up to expectations but even then the last and permanently-Beta-status GPS update was a lot better but too late to save the controller from obsolescence (is that a word?)
- Mikrokopter FC 2.1....good for up to 4S LiPo packs and best when used with MK's motor controllers (BL 2.0's), this is a system that is very reliable (if you make it past the first two or three test flights) and has very good GPS capabilities. In fact, Mikrokopter has been delivering excellent GPS features since before almost all of the other controllers have even been around! Mikrokopter is a challenge to set up compared to something like a NAZA but the results can be very satisfying. I'm still flying my first MK FC2.1 (purchased Nov 2010) on a coaxial XY-8 heli that can sit on the basement floor for months and then get called into service for a photo shoot or other mission and it does a great job!
- older APM versions, most will take the latest firmware although you should double check that claim before purchasing, processing speed/power was limited when managing eight motors but still very capable systems for most all hobby projects.
- DJI Wookong, a very robust and reliable flight control system, so much so that some pros are still using only Wookong controllers. Not fully expandable to work with DJI's latest camera gimbals but that may or may not matter to you in the long run.
- SuperX from Xaircraft, a good system that is in use by a lot of people. XAircraft should have been a major competitor to DJI but they just couldn't get their worldwide marketing and distribution systems into place as DJI did so DJI won. Ask around about the SUperX before you buy a used one, I'm not sure what the current status of development is with this system.
There are a lot of options out there! Don't just use NAZA now because I used it two years ago. It's still a good choice but so are a lot of other systems.
If you're a future FPV racer, try the NAZE32 or SP Racing F3 boards with this first build. If you're a tinkerer, try the APM 2.6 and figure out how to add an active sonar module to your heli, or an optical flow module for position hold indoors where GPS isn't available.
Whatever you do, HAVE FUN and prepare to be amazed!