That will depend on your comfort with electronics and what you want out of your FPV experience.
if you're going more stable than agile in your flight:
- the Naza boards are right at the top, with a price to match. Their lite board isn't crazy expensive but really all it will do is fly -- it won't manage a gimbal for you at the same time. Setup require very little electronics knowledge.
- 3DR's boards (APM, Pixhawk) are next in line , at a lower price, but will take a bit more setup and electronics know-how to get them happy.
If you're going more agility than stability:
- The Naze (yeah, not Naza) is one of the most popular, and very affordable, but requires quite a bit of setup -- you will need to *at least* solder on pin headers, and there's a bit of config to do with the board plugged into a PC. There is a version that can auto-tune, but I'd call it a mature experimental version at this point. (it works, but I wouldn't recommend it to a new builder . . . yet)
- Mulitiwii boards, like the flip boards, are slightly lower performance, and if the shipped ROM was setup, it has a good chance they'll work passibly well right out of the box, but expect a little PC setup -- you'll need to at least get the transmitter throws set via a PC. Both can fly incredibly well, but will need to be tuned to get above mediocre, which tuning a full PID is an art in itself.
- The KK2's are becoming a fairly old board, but simple to setup and tune, and if you buy a reflashed board (RTFQ sells a clone with a good image) won't need to see a PC -- you could do all the setup on the field . . . if you don't you'll want to pick up a USBASP module and reflash it pretty soon -- hardest part of that is installing the USBASP's drivers. There is about 1/2 hr of stepping through the menus and testing to get everything right, but plenty of walkthroughs that make it dirt simple. Since it only has a PI controller it won't fly as precisely as a tuned multiwii/Naze board but it's cook-book simple to tune a PI . . . and will fly far better than any untuned board.