Personally, I think ATI makes the best GeForce cards ;-).
Actually, pretty much every manufacturer follows the reference design very closely (even closer with the GF4's, with strict regulation of the number of capacitors on the end), and its the little things that make it stand out. That said, Leadtek, Gainward, ASUS, MSI, and even PNY all make fine cards (and I'm definately forgetting someone). I'd say the Gaindward cards are the most robust for overclocking (and those nice pre-overclocked "Golden Sample"), with the new Leadtek design for the GF4's being a close second, but you can't really go wrong with the current batch of cards.
I find it funny how Gainward uses the euphemism "Golden Sample" instead of simply "We overclock our cards ourselves and break the taboo that other manufacturers shun and call 'evil', 'warrantee voiding' and even 'amoral' ".
I never got that, calling overclocking amoral. Yeah sure, and I bet stealing cable is amoral too. Oh, wait a minute...