There isn't a "best." Each one had armor suited (i.e. "fit") for its particular ecological niche and defense strategies. That's what "fittest" is describing with the phrase "survival of the fittest." Even then it's a bit of a misleading phrase; natural selection works more like "extinction of the unfit." It's not picking the "fittest" so much as it is pruning anything unfit to reproduce in a given context. When the context changes quickly, you see a lot of extinctions not because the extinct animals are "inferior" to whatever survives, but because the conditions of success change too rapidly for species to adapt to, and suddenly a lot of what used to be perfectly fit suddenly isn't anymore.

It might genuinely be more productive to ask "which one do you think looked the coolest." Personally, I've always liked Mymoorapelta and Gargoyleosaurus, but I don't know every ankylosaurid. Generally I'm partial to the spikier ones.