Formal:
* Я скучаю по тебе. (Ya skuchayu po tebye) - This is the most common and literal translation of "I miss you." It is appropriate for all situations, including formal ones.
* Мне тебя не хватает. (Mne tebya ne khvataet) - This translates to "I lack you" and is a more poetic way of saying "I miss you."
Informal:
* Я скучаю. (Ya skuchayu) - This is a shorter and more casual way to say "I miss you." It's appropriate for friends and family.
* По тебе скучаю. (Po tebye skuchayu) - This is another casual way to say "I miss you." It's a little more affectionate than "Ya skuchayu."
* Соскучился/соскучилась. (Soskuchilsya/soskuchilas) - This is a very casual and informal way to say "I miss you." It's appropriate for close friends and family.
In addition to the above, you can also use phrases like:
* Я очень по тебе скучаю. (Ya ochen po tebye skuchayu) - "I miss you very much."
* Я сильно скучаю по тебе. (Ya sil'no skuchayu po tebye) - "I miss you a lot."
* Я по тебе соскучился/соскучилась. (Ya po tebye soskuchilsya/soskuchilas) - "I miss you a lot" (more casual).
The best way to choose the right phrase is to consider the context and your relationship with the person you are talking to.