There is no single most important ion. Without sodium, potassium, or calcium, you die. Sodium and potassium are necessary for nerve and muscle action potential conduction. Calcium is necessary for skeletal and cardiac muscle contraction. Basic physiology textbooks are good references.
The ions that are essential for normal biological functioning in the human body fall under four main group elements namely, sodium, potassium, magnesium, and calcium. There are also six d-block metal elements namely manganese, iron, cobalt, copper, zinc, and molybdenum that play important roles in the human body.
Although potassium ion is present maximum in the normal human body, calcium is an important ion in the human body as it has a vital role to play in various body functions. Besides building strong bones and teeth, calcium ions are involved in regulating almost all biological functions of the body, such as the heart and muscle contractions, neuro-information transmission, learning and memory, embryo formation and development, and cell proliferation. Calcium channels mediate calcium ions into the cytoplasm and organelles. Having too much or too little calcium in the blood can cause a wide range of symptoms across different systems in the body.
Attached below are a few references related to the important roles of calcium in the human body.
Article [The importance of calcium ions in the body]
Article Calcium ion is the key regulator of human sperm function
Article Calcium Ion – The Key Player in Cerebral Ischemia