Kingdom Animalia
The Kingdom Animalae contains many different species of animals. Their systems are complex because they require instincts and senses in addition to body parts.
Protista
The Protista Kingdom contains the most complex species as there is no such thing as a "typical" protist, it is filled with the "misfits" that cannot be categorized in any other kingdom.
Most Complex Structure in the Universe is Your Brain
The knee joint is likely the most complex in the body, followed by the shoulder joint.
The most complex tissue in the body is the musle.
Brain tissue
Animalia-- animals
Heart
The brain.
Structures and Features
Except for the brain, humans have a body that is as complex as any other mammal's.