You will get almost as many different opinions as replies. I assume you are looking to get a lens or more. The focal length would be influenced by what you intend to photograph. If you are looking to purchase one, my suggestion is to test it with a roll or two at different settings preferably with your camera on a tripod. Process the film and see the results. It would also be wise to purchase from a reliable source with a return policy.
I have five Hasselblad lenses and their 2x and am quite pleased with all.