More informal:
* Headstrong: This implies a strong will and determination, but can also suggest a lack of flexibility.
* Pig-headed: This is a more negative term, suggesting someone is unwilling to listen to reason.
* Obstinate: Similar to "pig-headed" but slightly more formal.
* Mule-headed: This refers to someone who is as stubborn as a mule, known for their stubbornness.
* Bullheaded: Similar to "mule-headed", implying a forceful and inflexible nature.
* Set in their ways: This implies someone is resistant to change and prefers their own habits.
* Stubborn as a mule/ox: This is a common idiom emphasizing the degree of their stubbornness.
* Hard-headed: This implies someone is unwilling to compromise.
More playful or humorous:
* Strong-willed: This can be used to describe someone who is determined and resolute, but can also be used in a more lighthearted way.
* Stick in the mud: This implies someone is resistant to change and prefers the status quo.
The best word to use will depend on the specific situation and your relationship with the person you're talking about.