Best camcorder under 500$ ?

Solution
The Panasonic camcorders you listed have larger sensors (1/2.3") than the Sony camcorders (1/5.8"). The larger sensor will give you better low-light performance. Of those listed I would go with one of the Panasonic models.
Can't speak for the first 3 cameras, but the GoPro uses a fixed lens and a smaller sensor. You're not going to be able to zoom with it (aside from digital crop and enlarge), and its low-light performance should be worse. The smaller sensor also means more depth of field. That can be a good thing if you're shooting lots of close-ups (e.g. video of bugs) or want the foreground and background to both be in focus all the time. A bad thing if you want the background to be blurred to emphasize the subject more.

Don't get me wrong. The GoPro is very nice and the company has made remarkable progress from the buggy crappy cameras they started out with. But it is simply a different type of camera than the first three you have listed. It's designed more for portability and withstanding abuse while sacrificing some functionality. You'll have to decide how much those tradeoffs and sacrifices will impact the types of video you wish to shoot.