Formal:
* Je pense à vous. (This is the most literal and formal translation.)
Informal:
* Je pense à toi. (Used for someone you know well, like a friend or family member.)
* J'ai pensé à toi. (More like "I was thinking of you", can be used for both friends and romantic partners.)
More romantic:
* Tu es dans mes pensées. (You are in my thoughts.)
* J'ai des pensées pour toi. (I have thoughts for you.)
The best choice depends on the context and your relationship with the person you are speaking to.