Drupal 6.0 ! No 6.x versions of or are out yet, so I'll have to hold off upgrading my sites a while longer.
I'm very exited about the new built-in multilanguage support. The old i18n contrib module didn't always work right with other modules since many module authors didn't take it into account.
I'm also looking forward to sinking my teeth into the improved theming system with the new .