Formal:
* मैं आपके बारे में सोच रहा/रही हूँ (Main aapke baare mein soch raha/rahi hoon) - This is the most literal translation and is appropriate for someone you don't know well or are trying to be respectful to.
Informal:
* मैं तुम्हारे बारे में सोच रहा/रही हूँ (Main tumhaare baare mein soch raha/rahi hoon) - This is a more casual way to say it and is suitable for friends or family.
* तुम मेरे दिमाग में हो (Tum mere dimag mein ho) - This means "You're in my thoughts" and is a more romantic way to express it.
Romantic:
* मैं तुम पर विचार कर रहा/रही हूँ (Main tum par vichar kar raha/rahi hoon) - This is a more poetic way to say "I'm thinking of you" and conveys a deeper level of affection.
* तुम्हारी यादें मेरे साथ हैं (Tumhaari yaadein mere saath hain) - This means "Your memories are with me" and is a very heartfelt way to express missing someone.
Note:
* The word "सोच रहा/रही हूँ" (soch raha/rahi hoon) is conjugated based on your gender. Use "रहा हूँ" (raha hoon) if you are male and "रही हूँ" (rahi hoon) if you are female.
* You can also use the pronoun "आप" (aap) for both formal and informal contexts. However, "तुम" (tum) is generally used in informal settings.
The best way to choose the right phrase is to consider your relationship with the person and the level of intimacy you want to convey.