It will vary depending on the domain name, but a one-word domain would be a good investment.
I used to think that .net was second after .com, but many people seem to thing the .org is more valuable. I think this is because the .net used to be a network (like ISP) provider, where .org was some kind of organization like a non-profit and possibly more "respectible". The .org was not connected to any type of industry, so more generic.