What is a key filling in a Crunchy Taco?

The key filling in a Crunchy Taco at Taco Bell is beef. This choice reflects the traditional and most popular ingredient used in their Crunchy Tacos. The beef is seasoned with Taco Bell's unique blend of spices, making it a distinct and recognizable taste that many customers associate with this menu item.

While other options, such as chicken and grilled vegetables, are available in different Taco Bell menu items, they do not define the Crunchy Taco itself. Fish is not part of the standard offerings for Crunchy Tacos, further solidifying beef as the staple filling. Thus, beef stands out as the quintessential ingredient that characterizes the Crunchy Taco at Taco Bell.