The most appropriate one-word substitution for the given group of words is adamant.

• Stickler means "a person who insists on strict adherence to rules or principles."
• Sticky means "difficult to remove or separate."
• Immature means "not fully developed or grown."
• Adamant means "firmly resolved or unyielding."

The word "adamant" is the best fit because it conveys the idea of a strong belief that is not easily swayed. It also suggests that the person is likely to expect others to follow their belief, as they are so strongly committed to it themselves.