I would have to say the best contributed module to Drupal is Views.
The best part of upgrading to Drupal 6.x is Views 2.
Views offer an easy way to display content. It is a query builder of content. There are so many cool things you can do with views and there are so many other modules that integrate with views. Not only that but you can export your views and put them in code to distribute to other sites.
Views make it really easy to theme your view and even provides you with theme information in your view so you know exactly what the name your template file.

Tips and Tricks (coming soon)
Altering field display in preprocess rather than using a template.
Using php to validate argument

Download Views module