Best way to migrate Content from drupal 6 to 7?
Migrate might be the module you need:


  The migrate module provides a flexible framework for migrating content into Drupal from other sources (e.g., when converting a web site from another CMS to Drupal). Out-of-the-box, support for creating core Drupal objects such as nodes, users, files, terms, and comments are included - it can easily be extended for migrating other kinds of content. Content is imported and rolled back using a bundled web interface (Migrate UI module) or included Drush commands (strongly recommended).


There is specialized Drupal-to-Drupal data migration for just the task you need:


  migrate_d2d provides a framework based on the Migrate API for migrating content from Drupal 5, Drupal 6, or Drupal 7 sites into the Drupal 7 site where it is installed. As of the 2.1 release, It requires Migrate 2.6 or later. Besides addressing contemporary needs to migrate to Drupal 7, it has helped serve as a proof-of-concept for incorporating the migration approach into core as an upgrade path (https://groups.drupal.org/imp).


It's far from perfect, all right, but it's good and well tested - in my experience it is most solid way unless you want to write custom one just for your case.