They are all correct to use.

However, this honestly depends on your voice and the listener/reader's perception.

For example, if you're crying like crazy while telling someone, "I miss you so much" or "I miss you so badly", then the meaning comes across exactly the same.

Same if someone asks you "how are you" and you reply "fine" or "I'm doing well" in the same tone of voice. It all depends on voice.

However, if I were writing this, I might say "badly" just because the correct use of the grammar might make the person think I'm serious, hahahaha.

I miss you bad. Is not very good English but is understandable.

I miss you so much. This is a the sort of description you might make to just about anyone you know well when you are away from them.

I miss you so badly. This is hightened emotion. So something you say to a lover of close family member. Often with an emphasis on "so".

They mean the same exact thing. I personally use "I miss you so much" because that's what I've been told the most. But if you have more friends that say "I miss you so badly/bad" then you might find that to be more fitting.

Regardless, it's up to your taste :)

I miss you so much or I miss you so badly have similar meanings. While I miss you so bad is often said, the grammar is incorrect.

"I miss you so much" this is the sentens that you need