It is difficult to determine which of the four Primarchs mentioned is the "best" or strongest, as they each have unique abilities, characteristics, and roles in the Warhammer 40,000 universe.
It's also important to note that the concept of "best" or "strongest" is subjective and can depend on the context and situation. That being said, I'll give you a brief overview of each of the four Primarchs mentioned and their notable characteristics.
Roboute Guilliman:
Also known as the "Avenging Son", he is the Primarch of the Ultramarines Space Marine Chapter and is known for his exceptional leadership, tactical brilliance, and diplomatic skills. He is considered one of the most capable military commanders in the Imperium and is revered for his ability to turn the tide of battle through his tactical genius. Guilliman is also known for his strict adherence to the Codex Astartes, a set of laws and regulations that govern the behavior and tactics of Space Marines, and his efforts to rebuild and reform the Imperium after the Horus Heresy.
Magnus the Red:
is the Primarch of the Thousand Sons Space Marine Chapter and one of the most powerful psykers in the Imperium. He is known for his extraordinary intelligence and mastery of the arcane and psychic powers of the Warp. Magnus is also highly respected for his wisdom and his role as an advisor to the Emperor, though he is sometimes considered unpredictable and irresponsible due to his reliance on divination and his tendency to act alone.
Mortarion:
Also known as the "Lord of Death", Mortarion is the Primarch of the Death Guard Space Marine Chapter and is known for his formidable strength, endurance, and resilience. He is a skilled warrior and a master of chemical and biological warfare, and his troops are known for their ability to resist poison, disease, and other forms of environmental hazards. Mortarion is also known for his hatred of the Imperium and his desire to bring about its downfall, as he believes it to be corrupt and oppressive.
Angron:
Also known as the "Red Angel", Angron is the Primarch of the World Eaters Space Marine Chapter and is known for his ferocious combat skills and insatiable bloodlust. He is a formidable opponent in close combat, his weapon of choice being a pair of massive axes known as the Gorefather and Gorechild. Angron is also known for his fury in battle and inability to control his anger, which often leads him to make impulsive and rash decisions.
In conclusion:
It is difficult to determine which of these four Primarchs is the "best" or strongest, as they each have their own unique abilities and characteristics. Guilliman is a skilled military commander and diplomat, Magnus is a powerful psyker and advisor, Mortarion is a master of chemical and biological warfare, and Angron is a ferocious fighter.
Ultimately, the decision as to which Primarch is the "best" or strongest depends on context and situation, as well as individual personality.
