It depends entirely upon the threat.
If you're talking about an Encase/Winhex/other forensics data gathering used on the drive, then one pass with all zeros will be sufficient.
If you're talking about physical examination of the original drive using the most advanced imaging techniques, then the more passes, the better. Gutmann would be the best in this case, simply because it uses so many overwrite passes. Whether it's overkill in terms of preventing some sort of data recovery (is 3 passes sufficient? 7 passes? that's not known), you'll have to decide for yourself.
Personally, I think if you're really that afraid of someone getting ahold of the drive who can bring rare techniques to bear, and you're that afraid that the data on the drive is overwhelmingly dangerous, then I'd be considering on the fly encryption of the drive contents, instead, and destroy the drive physically rather than overwrite/sell it when you're done.