Both Western Digital and Seagate are good. Some like WD more; others like Seagate. For what it's worth, they're pretty much equal. I personally prefer Western Digital.
For a primary drive, I suggest Western Digital Caviar Blue or Western Digital Caviar Black. The difference between Blue and Black is that Blue has 2 year warranty instead of the 5 years for Black. Seagate Barracuda is also good.