In Pardot, the best tool to use for assigning prospects to a group of users based on specific criteria is an automation rule with rule groups. Automation rules allow for the creation of complex criteria that prospects must meet to trigger certain actions. In this scenario, the rule would check for prospects with a score greater than 100, a grade greater than B, completion of any form on a landing page, and a "Product of Interest" field value of either A or B. Once a prospect meets these conditions, the automation rule can then assign them to the specified group of users. This method is efficient for processing large numbers of prospects automatically and continuously, which is not as feasible with completion actions on forms or landing pages, or with a one-time use segmentation rule.