First in priority is correct. Priority is a much-misused word. It is something to have, or to be in, not something to be. Better speakers never say something is a priority, but rather that it has priority.
"First", can be multiple things.
It can be an noun,
Ex: He was first in line
It can also be an adjective
Ex: He was the first man on the moon.
Finally, It can also be an adverb
Ex: First, we have to buy marshmallows, then we can make s'mores.
First can be a noun, adjective, and a adverb.
"First and foremost" is an adverb so the concept of grammar does not apply until you use it in a sentence.
The word 'first' is an adjective, a word that describes a noun as foremost in time or order.
He agrees with you. Or he is in agreement with you.
"That was wrong" is a grammatically correct sentence.
Yes, it is grammatically correct.
Yes, Almighty God is grammatically correct.
This sentence is grammatically correct.
No, I think the correct way is- Is this the first time you've seen it?
"Not like that" can be grammatically correct, depending on the context.
Yes, 'for free', is grammatically correct.
Yes! That is grammatically correct!
The phrase "Is you don't miss me do you" is not grammatically correct. It should be rephrased to something like "Don't you miss me?" to be correct.
He agrees with you. Or he is in agreement with you.
Needs to be a grammatically correct question!
The phrase "You are not knowing" is not grammatically correct. Instead, you can use "You do not know" or "You are not aware."
Yes, it is grammatically correct.
Yes, Almighty God is grammatically correct.
"That was wrong" is a grammatically correct sentence.
The sentence "Rhoda's Crazy" is not grammatically correct. It is missing a verb to make a complete sentence.