so fun/so much fun

I'd go with the 2nd

Both are perfectly fine and natural.

This is fun. This is so fun. This is so much fun. This is so much freaking fun. Each one is more fun than the one before it.

"so much fun" is more common. Some people say "so fun", especially in American English. It's colloquial/slang.

So fun is using fun as an adjective. So much fun is using fun as a noun. both mean the same thing, it's just whether you're modifying a noun or using the copula to describe a noun with another.