best version -> 5.x or 6.x?

2nd that-- if starting a new project, I'd recommend comparing your particular project needs list with the available/stable Module list for either Drupal 5.x or 6.x. Since Drupal functionality is so extendable through modules, IMO the decision is often more about "is module XYZ that I need available and stable" vs. core, so determining your module needs will actually determine which core version you need, unless of course most of your needs are met with core or project growth plan is adequate enough to wait for 6.x modules to become available.