Depends a little on what you're backing up. If it's just a single computer, then I like the WD or Seagate drives. I have a seagate connected over usb to my iMac right now. But, as we have a house full of Macs, we use a couple of time capsules for backup because we can get to them wirelessly and reasonably pain free.
You might want to also consider something to keep off site as well.