If you're talking about one of, if not the best, choices for the most defensive Cookie in Cookie Run Kingdom, then the Ancient rare Hollyberry Cookie should definitely be your first choice.
Hollyberry Cookie Beascuit & Topping Guide | Cookie Run Kingdom
The Ancient Cookies in Cookie Run Kingdom play a huge role in the game's story, as they're the five who started it all. But their weight in the story is just as important as their combat skills, and Hollyberry Cookie is one of the most reliable choices for your most defensive Cookie.
Here's everything you need to know about Hollyberry Cookie, including the best toppings guide you can use for her in Cookie Run Kingdom.
Since Hollyberry Cookie is a Front and Defense-type Cookie, the best toppings you can use on her are five Solid Almond toppings. By doing this, you can increase her damage resistance by five percent for each. This will allow her to better defend her teammates while also contributing to the offense by activating the Berry of Life as often as possible.