It depends. If you want a small one, I think my Western Digital (WD) Passport has served me very well. You don't need a power source, and you only need one cable to plug into your MacBook's USB. My other backup, an older Seagate with FW, just sits there. Actually, I just put it away. The AC adapter cable is attached to the power brick. A 2nd cable is needed to connect the power brick to my harddisk in order to power it. That's cable --> power brick --> harddisk. Another FireWire cable is needed to attach the drive to my computer.
That's too much! It's also bulkier than my WD harddisk. Sure, you pay more money per GB of space, but the convenience is so much better that I actually back up more often. I may even get a 2nd WD Passport drive.
If you're going to get a full-sized drive, the Seagate FreeAgent 500 GB drives are good and cheap.