Formal:
* איך מיז דיר: (Ikh mis dir) - This is a direct translation and is the most formal way to say it.
* איך פעל דיר: (Ikh fel dir) - This literally means "I am missing you" and is also formal.
Informal:
* איך ווער דיר געמישט: (Ikh ver dir gemisht) - This means "I am getting mixed up about you" and is a more affectionate way to say "I miss you."
* איך האב דיר געמישט: (Ikh hob dir gemisht) - This means "I have missed you" and is also a more affectionate way to say it.
* איך געפינ דיר: (Ikh gefin dir) - This means "I am missing you" and is a more casual way to say it.
Even more affectionate:
* איך האב דיר אויף דעם הארץ: (Ikh hob dir af dem harts) - This means "I have you on my heart" and is a very loving and heartfelt way to say "I miss you."
The best way to choose which phrase to use depends on your relationship with the person you are talking to and the context of the situation.