【Obstinate】 , 【dogged】 ,  【stubborn】 ,  【pertinacious】 ,  【mulish】 ,  【stiffnecked】 ,  【pigheaded】 ,  【bullheaded】  are comparable when they mean fixed or unyielding by temperament or nature.

【Obstinate】 implies persistent adherence, especially against persuasion or attack, to an opinion, purpose, or course; when applied to persons or to their ideas or behavior the term often suggests unreasonableness or perversity rather than steadfastness.

【Dogged】 adds the implication of downright and tenacious, sometimes sullen, persistence; usually, also, it connotes great determination or an unwavering purpose.

【Stubborn】  is often used interchangeably with  【obstinate】  and  【dogged】 , for it implies the unyielding adherence of the one and the tenacious determination of the other; more strongly than either of them, however, it carries an implication of a native fixedness of character or of a deeply rooted quality that makes a person sturdily resistant to attempts to change his purpose, course, or opinion, or that makes a thing highly intractable to those who would work it, treat it, or manipulate it.

【Pertinacious】  lacks, as compared with  【obstinate】 , the implication of resistance, and as compared with  【stubborn】 , the suggestion of inherent quality; it usually implies a chosen course and stresses its pursuit with stick-to-itiveness and, often, with a persistence that is annoying or irksome.

【Mulish】 suggests an obstinacy as characteristic or as unreasonable as that of a mule.

Stiff-necked , more even than  【obstinate】  or  【stubborn】 , stresses inflexibility; it often also suggests a haughtiness or arrogance that makes one incapable of respecting the commands, wishes, or suggestions of others.

【Pigheaded】  and  【bullheaded】  suggest a particularly perverse or stupid kind of obstinacy; therefore they are chiefly terms of severe reproach;  【pigheaded】 , however, often suggests impenetrability to argument and  【bullheaded】 , headstrong determination.