Version 8 is the good to go at the moment. Version 9 will be released soon, but take some time to get stable.

odoo v8: Is the LTS (Long-Term Stable) version that is updated with bug patches and is wholey maintained by odoo with community submissions, however it does not receive new features/functionality.

saas-6: Is the first minor update of odoo 8 that includes bug fixes and will include new "stable alpha" features that will become available in the next LTS version of odoo. Only odoo develops a migration path between saas versions, the community does not. This version is available to odoo's online web saas customers when it is released but can be rolled back or migrated to the next saas version at odoo's discretion.

OCB v8: Is a version of odoo 8 that is maintained by the OCA (Odoo Community Association), to permit them to respond to a community developed patches faster than odoo. All updates must be submitted to the official odoo branch for adoption in the official odoo release before they can be considered for the OCB version. Like odoo v8 only bug fixes are accepted and migration to the next LTS version is possible through either odoo or the community migration scripts.