Formal:
* My favorite subject is...
* I enjoy the subject of... most.
* My preferred subject is...
Informal:
* My fave subject is...
* I really love...
* I'm all about...
You can also use phrases like:
* I'm most interested in...
* I'm passionate about...
* I excel in...
For example:
* My favorite subject is history.
* I really love math.
* I'm most interested in science.
Choose the phrase that best suits the context and your personal style.