In French, you say “mon préféré” if you’re referring to a masculine noun, and “ma préférée” if it’s a feminine noun. This is because French is a gendered language, meaning that nouns have a gender attached to them. For example, if you want to say ‘my favorite book’ (which is masculine in French, ‘livre’), you would say ‘mon livre préféré’. On the other hand, for something feminine like ‘my favorite movie’ (which is ‘film’), it would be ‘ma film préférée’.
It’s also worth noting that if you want to say ‘my favorite things’ in a general sense, you can use the plural form, where it becomes ‘mes préférés’ for masculine plural or ‘mes préférées’ for feminine plural. Understanding and using gender correctly is key to speaking French well.