depends on brand, size and speed.
first i would get a well known HD brand like WD or seagate or samsung.
then i would decide on the size i wanted: 40 gig, 80 gig, 100 gig, 250 gig, w/e you need.
then speed, which is a big factor, 5400rpm are common among external hDs, but i would lean toward a 7500rpm drive like a regular internal one.