WD and Seagate are my first choices for external hdds. I trust both equally, so it really comes down to price.
Amazon is always a good place to price check, even for computer parts. Most of their products incur free shipping on orders over $25 (which your external hard drive is sure to exceed)
Main: i5-3570K, ASRock Z77 Pro4-M, MSI RX480 8G, 500GB Crucial BX100, 2 TB Samsung EcoGreen F4, 16GB 1600MHz G.Skill @1.25V, EVGA 550-G2, Silverstone PS07B
HTPC: A8-5600K, MSI FM2-A75IA-E53, 4TB Seagate SSHD, 8GB 1866MHz G.Skill, Crosley D-25 Case Mod