If you can not have .com, it is always recommended to have .net or the country level domain such as .us or .in, etc.
Yeah, I've never been to a .biz website. To me, it's .org after .com. .ORGs are usually available (compared to .net) and carry an air of legitimacy you lose with other extensions.
I would say .net after .com the and possible then .info i have never really thought about the others but if i had to choose from the lesser used ones i would think .org