Completist

From Teletraan I: The Transformers Wiki

A completist is a fan who collects a complete set in some form. Some completists attempt to have a full set of Generation 1 or Beast Wars figures, while others simply collect all versions of a single character (Optimus Prime or Hot Shot) or a specific subgroup or subline.

Exactly what makes up a "full set" varies from fan to fan; some might settle for a single loose version of each individual character, while others might want complete, boxed versions, while others collect packaging variants or accessory variants.