I recommend a WD Passport because they're small, they don't require their own power source (ie: they're powered by USB), and they're easily removed and put away somewhere. If you don't like Western Digital (WD), then buy a Seagate one, or whatever brand of HDDs you trust. In fact, buy 2 or 3 of them, and keep one at work, one at home, and one at your parents' home. If you buy the low capacity drives, they'll be cheap, small, portable, and easily carried off-site. These drives are simply easier to keep off-site, which is the most important thing about backing up. After all, keeping everything backed up, but in the same room may not help you much. What if you get robbed? What if there's a fire in your house. Also use flash drives if you want. They're cheap, and it's much more difficult to physically break down a flash drive than an optical one. Again, they're easy to back up to and carry away.
I don't know about you, but the reason I don't like my full size, 3.5" backup HDD as much as my WD Passport is due to all of these things.