Western Digital Caviar Black. Best bang for your buck without sacrificing performance. I buy them in bulk and drop them into a rack data cluster. Never had one fail me yet.
If they're for infrequent backup use, go with the Western Digital Caviar Green. There's a performance hit, but lower priced than the Blacks.
Edit: these are OEM drives, not portable enclosures. Just occurred to me you're probably looking for something in a case.