Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
Any drive will work. There are so many good brands and models to choose from that it's hard to give you a recommendation without being almost all-inclusive. The one thing to look for is a bus-powered drive if you want something portable, but some drives may require that you use two USB ports to supply enough current. Other than that, just pick a model that has good reviews and suits your budget.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.