HGST /thread
Seriously though, when I bought new HD's for my server, I wanted something that could be on 24/7 and a brand that I trusted with great reviews. I settled on HGST and haven't looked back since. I personally also like WD drives and I keep hearing good things about Toshiba drives, although I've never personally had one. I also read a lot and have seen a fair amount of Seagate drives fail, idk what it is but I have a bad feeling about them so I steer clear.
As far as size, 4tb seems to be a pretty good place to start with prices going up a good amount from there.