I use a WD MyBook Premium 500g and it is fantastic. The reason I went with the MyBook is because I can use it as room decoration (I have it sitting on my book shelf), it is passive cooling (No noise at all), it comes with its own backup software, and it was cheaper than any other brand out there.
However, from what I have seen, the top three external HDs are Western Digital, Seagate, and Lacie.