Formal:
* Я думаю о тебе. (Ya dumayu o tebye.) - This is the most literal translation and is suitable for formal situations.
* Я о тебе вспоминаю. (Ya o tebye vspominayu.) - This means "I remember you" and implies that you're thinking about someone specifically.
Informal:
* Я о тебе думаю. (Ya o tebye dumayu.) - This is a more casual and common way to say it.
* Я тебя вспоминаю. (Ya tebya vspominayu.) - This is a more informal version of "I remember you."
* Думаю о тебе. (Dumayu o tebye.) - This is a very short and informal way to say "I'm thinking of you."
Romantic:
* Я тебя люблю. (Ya tebya lyublyu.) - This means "I love you" and can be a very romantic way to say "I'm thinking of you."
* Я скучаю по тебе. (Ya skuchayu po tebye.) - This means "I miss you" and implies that you're thinking of someone and wishing they were with you.
Other:
* Ты у меня в мыслях. (Ty u menya v myslyakh.) - This means "You are in my thoughts."
* Я не могу выбросить тебя из головы. (Ya ne mogu vybrosit tebya iz golovy.) - This means "I can't get you out of my head."
The best way to say "I'm thinking of you" in Russian depends on the specific situation and your relationship with the person you're talking to.