* Most abundant species: The crabeater seal is the most abundant seal species, with an estimated population of 15 million individuals.
* Most widespread species: The harbor seal has the widest geographic distribution, found in coastal waters of the North Atlantic and North Pacific Oceans.
* Most frequently encountered by humans: This would likely be the harbor seal again, as it's often seen in coastal areas and harbors.
To answer your question accurately, please clarify what you mean by "most common."