What If? A Primarch becomes an Avenger at the start of the first Avengers.

[Tyzuris]

And to ellaborate further, the Primarch doesn't replace any character, so they're an addition to the team. Also assume that they're willing to work with Avengers in the goals and objectives they are trying to accomplish, but assume the Primarch's character, etc... plays a factor into how they behave and interact with the team. So Perturabo for instance can be a cold, callous and remote and unrelatable (+ bitter) member of the team.

And for this what if - assume you can make predictions with each of the Primarchs. Assume they get their canon wargear and ability to maintain it.

 

[Scooby Doo]

The Primarchs basically wins everything up until Infinity War and they only lose if they don't go for the blitz.

 

[Spartan303]

A Primarch is seriously overpowered in such a setting.

 

[The Whispering Monk]

Well, not much is gonna challenge the Primarch.

The challenge is going to be keeping the Primarch from just taking over the planet!

 

[PsihoKekec]

Avengers need Batman, so Conrad it is.

 

[The Whispering Monk]

Thor could use a drinkin' buddy...Russ would go well here. Maybe show Thor how to be King.

 

[Vyor]

I will offer a correction: there is someone on the Avengers that can best an Avenger physically. Hulk. Thor might be able to do it as well, depending on the exact primarch.

Also I vote Vulkan!

 

[Tyzuris]

Also I vote Vulkan!

His perpetuality would be funny. Thanos snaps Vulkan away and moments later he reappears leading Thanos in WTF state.

Despite Magnus being in the top three most powerful psykers list being able to do a wide variety of things with his powers, I would say Vulkan with being unable to be permanently killed to be the most powerful Primarch.