Formal:
* Я люблю тебя (Ya lyublyu tebya) - This is the most common and literal translation, suitable for formal situations or when addressing someone older or in a position of authority.
Informal:
* Я тебя люблю (Ya tebya lyublyu) - This is the more common informal version, used among friends, family, and loved ones.
More affectionate:
* Я люблю тебя очень (Ya lyublyu tebya ochen') - "I love you very much"
* Я люблю тебя сильно (Ya lyublyu tebya sil'no) - "I love you deeply"
* Я тебя обожаю (Ya tebya obozhay) - "I adore you"
Other ways to express love:
* Ты мне очень дорог(а) (Ty mne ochen' dorog(a)) - "You are very dear to me"
* Ты моя(й) любимый(ая) (Ty moya(y) lyubimy(aya)) - "You are my beloved"
The best choice for you will depend on the situation and the person you are speaking to.