Formal:
* Je suis amoureux/amoureuse. (This is the most direct translation and is appropriate for formal situations.)
Informal:
* Je suis amoureux/amoureuse de toi. (This translates to "I am in love with you" and is more personal and intimate.)
* Je t'aime. (This means "I love you" and is the most common and heartfelt way to express love in French.)
Other options:
* J'ai des sentiments pour toi. (This means "I have feelings for you" and is less intense than "I'm in love.")
* Je suis fou/folle de toi. (This means "I'm crazy about you" and is a more playful and lighthearted way to express love.)
The best way to say "I'm in love" in French depends on the context and the relationship between the speakers.