I have used 2TB Samsung F4 succesfully with my Playon!Hd2 but i need new hdd now. Im tempted to buy 3TB WD green. What do you guys think, is it reliable and work well with pohd2?

My computer do have win7 64-bit and asrock z68 board, so it will be on my computer.

So:

F4 2TB
Barracuda Green 2TB
WD Green 2TB
WD Green 3TB

which one would you get?

Officially AC Ryan support 2TB drives, as long as they are externally powered.
However you can use 3TB drives and Arne has uploaded a guide on how to use them here.
Can't comment on the individual drives - I'd be looking for speed at a decent price, but would read the support forums first to check
Andy